Prevent WiredTiger read ticket count from going negative

ISSUE SUMMARY
MongoDB 3.4.12 contains a bug that may cause WiredTiger read ticket count to become negative.

ISSUE IMPACT
Users may observe erroneous metric reporting. In particular, read queue metrics, which are derived from read tickets, will appear much larger than they actually are.

The default limit of 128 available read tickets becomes effectively larger, thus allowing more readers into the system. Workloads which hit the maximum read ticket limits and rely on this mechanism for back pressure may suffer performance degradation.

AFFECTED VERSIONS
Only MongoDB 3.4.12 is affected, and only when using the WiredTiger storage engine.

FIXED VERSIONS
A fix is included in MongoDB 3.4.13.