Add alloc_hooks API to mock server

To allow tests using the mock server to make use of the allocator
hooks API, expose them through the mock server.

Change-Id: Id8da53dd812ded1b6aaa06861690400482099ee9
Reviewed-on: http://review.couchbase.org/41488
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>