Free 'custom-allocated' buffers

Buffers too big to fit inside an allocation chunk were not freed,
resulting in serious memleaks when large buffers were allocated

Change-Id: I213b35185ed8eff1312fd09cdb62206c8251e69d
Reviewed-on: http://review.couchbase.org/28962
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>