MB-11067: Allocate ilist on demand.

The itemlist member of the connection object is only used for TAP/UPR
connections. Therefore stop allocating memory for it up front for all
connections, and instead just allocate when a connection first tries
to use it.

Change-Id: Ia3ebeb94154d5f3ef127f26c901b10a744133c34
Reviewed-on: http://review.couchbase.org/38654
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>