MB-11066: Remove up-front connection allocation, allocate on-demand

Instead of defining a statically-sized array of maximum number of
connections, simply allocate connections on demand when needed. Add an
intrusive linked-list to track them (for destruction purposes). Also
remove the pre-allocation of connections.

This paves the way for supporting dynamic modification of the maximum
number of connections.

Change-Id: I7a675ecc940f5cfb357064dc2c1c2d3b8057796d
Reviewed-on: http://review.couchbase.org/39018
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>