Reduce ARC in HPACK closestMatch

[Lukasa]

Motivation:

When analysing remaining performance cliffs, we observed that
HeaderTableStorage.closestMatch was the source of a surprising amount of
ARC traffic. Further diagnosis revealed that we were ARCing string
storage as we iterated the table. Given that the table is constructed of
String/String pairs, the effect would be that if we scanned a table we
would emit two retains and two releases for each entry. Whenever we
encode a HTTP/2 header that we think could be indexed, we will scan the
static and possibly the dynamic tables to find a header that matches.
This ends up being a pretty nasty performance cost, as swift_retain and
swift_release are particularly complex on Strings, and often have
trouble with cache coherency and cache misses.

This got root-caused to https://bugs.swift.org/browse/SR-13931.

Modifications:

• Work around SR-13931 by avoiding using the iterator.
• While we're here, remove generics as well, as we aren't using them.

Results:

4% perf gain in 10k requests 100 concurrent streams benchmark and
10k requests 1 concurrent streams benchmark.