Pre-allocate res change-items based on statistic

[laskoviymishka]

Calc avg row size so we can pre-allocate change-item slice.

Before:

BenchmarkGenericParser
BenchmarkGenericParser/metrika
BenchmarkGenericParser/metrika/batch_size_1
BenchmarkGenericParser/metrika/batch_size_1-10         	   42182	     27948 ns/op	 146.38 MB/s	   40584 B/op	     321 allocs/op
BenchmarkGenericParser/metrika/batch_size_5
BenchmarkGenericParser/metrika/batch_size_5-10         	    2776	    429816 ns/op	 152.36 MB/s	  650278 B/op	    5020 allocs/op
BenchmarkGenericParser/metrika/batch_size_10
BenchmarkGenericParser/metrika/batch_size_10-10        	      81	  13386086 ns/op	 156.55 MB/s	20820405 B/op	  160525 allocs/op
BenchmarkGenericParser/taxi
BenchmarkGenericParser/taxi/batch_size_1
BenchmarkGenericParser/taxi/batch_size_1-10            	   30919	     36305 ns/op	  54.43 MB/s	   35698 B/op	     385 allocs/op
BenchmarkGenericParser/taxi/batch_size_5
BenchmarkGenericParser/taxi/batch_size_5-10            	    2134	    565053 ns/op	  56.01 MB/s	  577095 B/op	    6004 allocs/op
BenchmarkGenericParser/taxi/batch_size_10
BenchmarkGenericParser/taxi/batch_size_10-10           	      63	  17794249 ns/op	  56.91 MB/s	18091908 B/op	  194000 allocs/op
BenchmarkGenericParser/mdb
BenchmarkGenericParser/mdb/batch_size_1
BenchmarkGenericParser/mdb/batch_size_1-10             	  130441	      9152 ns/op	  74.63 MB/s	    7584 B/op	     103 allocs/op
BenchmarkGenericParser/mdb/batch_size_5
BenchmarkGenericParser/mdb/batch_size_5-10             	    8551	    138940 ns/op	  78.87 MB/s	  123364 B/op	    1544 allocs/op
BenchmarkGenericParser/mdb/batch_size_10
BenchmarkGenericParser/mdb/batch_size_10-10            	     274	   4359217 ns/op	  80.45 MB/s	 3920205 B/op	   49315 allocs/op
BenchmarkGenericParser/json-sample
BenchmarkGenericParser/json-sample/batch_size_1
BenchmarkGenericParser/json-sample/batch_size_1-10     	   17776	     68195 ns/op	  95.55 MB/s	   93827 B/op	     986 allocs/op
BenchmarkGenericParser/json-sample/batch_size_5
BenchmarkGenericParser/json-sample/batch_size_5-10     	    1160	   1024875 ns/op	 101.75 MB/s	 1471232 B/op	   15896 allocs/op
BenchmarkGenericParser/json-sample/batch_size_10
BenchmarkGenericParser/json-sample/batch_size_10-10    	      34	  32322842 ns/op	 103.25 MB/s	52604432 B/op	  515963 allocs/op
PASS

After:

BenchmarkGenericParser
BenchmarkGenericParser/metrika
BenchmarkGenericParser/metrika/batch_size_1
BenchmarkGenericParser/metrika/batch_size_1-10         	   42922	     28136 ns/op	 145.40 MB/s	   40587 B/op	     321 allocs/op
BenchmarkGenericParser/metrika/batch_size_5
BenchmarkGenericParser/metrika/batch_size_5-10         	    2733	    432692 ns/op	 151.35 MB/s	  645956 B/op	    5016 allocs/op
BenchmarkGenericParser/metrika/batch_size_10
BenchmarkGenericParser/metrika/batch_size_10-10        	      82	  13368213 ns/op	 156.76 MB/s	20646278 B/op	  160527 allocs/op
BenchmarkGenericParser/taxi
BenchmarkGenericParser/taxi/batch_size_1
BenchmarkGenericParser/taxi/batch_size_1-10            	   32324	     36165 ns/op	  54.64 MB/s	   32783 B/op	     382 allocs/op
BenchmarkGenericParser/taxi/batch_size_5
BenchmarkGenericParser/taxi/batch_size_5-10            	    2126	    558261 ns/op	  56.69 MB/s	  522986 B/op	    5997 allocs/op
BenchmarkGenericParser/taxi/batch_size_10
BenchmarkGenericParser/taxi/batch_size_10-10           	      63	  17978731 ns/op	  56.33 MB/s	16610718 B/op	  193980 allocs/op
BenchmarkGenericParser/mdb
BenchmarkGenericParser/mdb/batch_size_1
BenchmarkGenericParser/mdb/batch_size_1-10             	  128125	      9239 ns/op	  73.93 MB/s	    7584 B/op	     103 allocs/op
BenchmarkGenericParser/mdb/batch_size_5
BenchmarkGenericParser/mdb/batch_size_5-10             	    8751	    139038 ns/op	  78.81 MB/s	  119005 B/op	    1540 allocs/op
BenchmarkGenericParser/mdb/batch_size_10
BenchmarkGenericParser/mdb/batch_size_10-10            	     272	   4506998 ns/op	  77.82 MB/s	 3750451 B/op	   49303 allocs/op
BenchmarkGenericParser/json-sample
BenchmarkGenericParser/json-sample/batch_size_1
BenchmarkGenericParser/json-sample/batch_size_1-10     	   19152	     62470 ns/op	 104.31 MB/s	   66309 B/op	     980 allocs/op
BenchmarkGenericParser/json-sample/batch_size_5
BenchmarkGenericParser/json-sample/batch_size_5-10     	    1216	    980603 ns/op	 106.35 MB/s	 1064461 B/op	   15884 allocs/op
BenchmarkGenericParser/json-sample/batch_size_10
BenchmarkGenericParser/json-sample/batch_size_10-10    	      36	  31956435 ns/op	 104.43 MB/s	33880404 B/op	  515899 allocs/op
PASS