Add timing_stress_for_test option for lookaside cleanup

The lookaside file is being actively used by MongoDB, and we have been making a lot of enhancements to it recently. One change has been enhancing the garbage collection of lookaside file, which introduced more interactions with other threads - that can lead to race conditions.

It'd be interesting to add a timing_stress_for_test option for lookaside file usage, and enable testing of it in Jenkins.