Synchronize when acking settings.

Without this tests fail.
Merge pull request #464 from square/jwilson_0119_fix_race

Synchronize when acking settings.
Change spdy and http/2 headers to always lowercase. Introduce allocation-free ByteString.equalsAscii.
Move more cache headers out of public API.
Merge pull request #462 from adriancole/bytestring-eqic

Change spdy and http/2 headers to always lowercase
Merge pull request #461 from square/jwilson_0119_more_cache_headers

Move more cache headers out of public API.
Drop obsolete imports.

These crept in after rebase.
Merge pull request #465 from square/jwilson_0119_imports

Drop obsolete imports.