[Perf] High Allocator MemoryPoolIterator2.GetString

[benaadams]

Drill down on Issue #288
Measurements post PR #287

Item 1 on highest allocators in plaintext test

Allocations (2003 requests)

Bytes (2003 requests)

Also the HTTP portion of the request is ASCII bytes rather than UTF8 or other encoding https://tools.ietf.org/html/rfc7230#section-3.2.4

Note: Path can be decoded to utf8 before being converted to string so may be utf8

[benaadams]

Reduced some; and ascii headers in #312 "Reduce GetString allocs and conversions"

Lazy string creation is possible as extension, but would get, unusual