Checkpoint only after Update; Keep records in the sync.Map longer

[jmacd]

Removes TODOs left from #468. An atomic increment is carried out for each Update(), allowing the collection logic to checkpoint records only when they have actually received updates (2 TODOs). This was mentioned in the review for #641.

This allows an optimization that had been in place before #468, that is to defer unmapping a record until it has survived a whole collection interval without being updated (1 TODO). This ensures that users of the direct calling convention that happen to repeatedly use the same label set will benefit from cached label encodings.

BEFORE:

BenchmarkRepeatedDirectCalls-8   	 1217017	      1001 ns/op	     304 B/op	       7 allocs/op

AFTER:

BenchmarkRepeatedDirectCalls-8   	 2575458	       454 ns/op	     240 B/op	       3 allocs/op

[krnowak]

Looks good, but there is a conflict to resolve.