folly::io::QueueAppender speedup

Summary:
Make QueueAppender (as used in the thrift2 protocol writer) much faster.
Benchmarks in a separate diff (for thrift2).

A few things:
- remove IOBuf range checks in optimized mode
- simplify QueueAppender: maxGrowth wasn't used, removed it
- simplify QueueAppender: don't duplicate the work in IOBufQueue, let
IOBufQueue keep track of memory
- speed up IOBuf::isSharedOne(): fast-path case where the buffer was never
shared (so don't read the reference count in that case)
- fast-path in QueueAppender if size is known at compile time

@override-unit-failures
compilation errors outside of folly fixed in subsequent diff

Test Plan: folly/io/test, both dbg and opt

Reviewed By: davejwatson@fb.com

FB internal diff: D946907
fix folly build on OSX with gcc 4.8

Summary:
- gcc 4.8 + boost 1.53 do not like the Oxford comma in template
arguments

Test Plan: compiled on OSX with gcc 4.8 and boost 1.53

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D949373