CCBC-159 Fix MockUnitTest.testPurgedBody

Update socket timer only if there was real job done like reading or
writing data.

Fixes http://sdkbuilds.couchbase.com/job/libcouchbase/406

Change-Id: I6415b7c6d7ba64ef0fdeb722e92ecc98c4208ae7
Reviewed-on: http://review.couchbase.org/23668
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>