Review internal session allocation accounting

As part of lookaside eviction enhancements, we added some additional internal sessions. We have a mechanism for tracking approximate internal session usage, so applications that configure small numbers of session handles don't run out due to internal usage - we should review whether that allocation is still reasonable, and if there is a more maintainable mechanism for tracking internal session usage moving forward.

The particular magic number that tracks internal session allocation is WT_EXTRA_INTERNAL_SESSIONS