Optimization: Fewer memory (re)allocations for HTTP headers (#239)

Optimization: Fewer memory (re)allocations for HTTP headers (#239)

Tests revealed multiple fresh memory allocations/deallocations while
storing small (few fields) HTTP headers. Many popular sites use larger
headers (15-30 fields). To avoid expensive memory operations:

1. Pool all std::vector<HttpHeaderEntries*> memory allocations.
2. Prevent reallocations (for HTTP headers with fewer than 32 fields).

This optimization deals with storing the header index. It does not
affect how individual header fields are stored.