Add RDB readbuffer system

The RDB system allows for pluggable allocators for efficient read
buffers. See the src/rdb/* and tests/rdb/* files for more details

Change-Id: I2cfc5b903cfce81f8d0fbd891527bddd3a76a3c8
Reviewed-on: http://review.couchbase.org/35867
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>