POPPY-0019: Number of headers in RequestParser/RequestBuffer

In request parser we should allocate the memory for a common number of HTTP request headers. In this way we could avoid the frequent reallocation of those headers.

Locate the RequestParser and RequestBuffer implementations and inspect how HTTP request headers are allocated and grown. Determine an appropriate common initial header capacity, then verify that parsing typical requests avoids repeated reallocation without breaking header handling.

Written by the indexing model from the issue text.