Faster dump by less tiny writes()

[staabm]

while writing the dump the script is doing millions of tiny writes().

after this PR we are buffering the contents within a string a do the write only after we reached the net-buffer-length.
this means we are doing waaaay less IO calls.

before this PR:

mstaab@mst22:/cluster/www/www/www/mysqldump-php$ time php test2.php

real    0m14.194s
user    0m10.762s
sys     0m2.340s

after this PR

mstaab@mst22:/cluster/www/www/www/mysqldump-php$ time php test2.php

real    0m7.492s
user    0m6.403s
sys     0m0.204s

which is now 40-50% faster 🚀

[staabm]

blackfire profile before this PR

after this PR (this slow write() path is completely gone)

the remaining escape slowness is handled in #34