MB-11064: Don't preallocate more than 5000 connections

This is a temporary change to avoid using too much memory when
we're significally increasing the maximum number of connections.
The default value of 5000 was chosen to keep the behavior as
in the earlier releases.

Moving forward this limit will be dynamic

Change-Id: I5155ce9c73b71dc6b9d5688ac57161a741c395e2
Reviewed-on: http://review.couchbase.org/36802
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>