Expand stats `allocator` buffer size

jemalloc prints longer detailed stats than tcmalloc, so increase the
buffer size to match.

Change-Id: Ic40fbac3cb06ed459eaa7140dce39aac81492625
Reviewed-on: http://review.couchbase.org/39873
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>