[#2642] CompositeByteBuf.deallocate memory/GC improvement

Motivation:

CompositeByteBuf.deallocate generates unnecessary GC pressure when using the 'foreach' loop, as a 'foreach' loop creates an iterator when looping.

Modification:

Convert 'foreach' loop into regular 'for' loop.

Result:

Less GC pressure (and possibly more throughput) as the 'for' loop does not create an iterator
Upgrade to HTTP/2 draft 13

Motivation:

Need to upgrade HTTP/2 implementation to latest draft.

Modifications:

Various changes to support draft 13.

Result:

Support for HTTP/2 draft 13.