MB-8083: decrease the number of threads in multiple_connections_using_memcachetest

 based on Steve's comment in the ticket

Change-Id: I395bdec45b19606092fa70078e92aa02de3f9d80
Reviewed-on: http://review.couchbase.org/37956
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>