do not initialize StringLibContext's static buffers

[apolyakov]

This pull request enhances the performance of the HelloWorld benchmark for K2 by a factor of 32. Previously, we used zero-initialized std::array as a buffer in StringLibContext. This zeroing of the 8MB array at the start of each request consumed significant CPU time. By switching to uninitialized std::array, we avoid unnecessary initialization, resulting in substantial performance improvements.

Following benchmark is enough to see the difference:

?php

echo "Hello, World!\n";

[mkornaukhov03]

OK, but let's run better internal tests. I'll give an approve after passing them.