minor refactorings - less synchronization around TS::Cache
support the :wait_timeout option + at least debug when `pool: size` not set
keep the same array connections instance - than Array#size is pretty much "atomic"
3.2/4.1 adapter compatibility for < 3.2
adapter compat layer aligned with default-pool (which now works under 3.x as well)
- fixed typo and removed unused set require
- not need to synchronize (expire, lease are always called from a synchronized block)
degrade truthy requirements for `in_use?` return value (due DefaultPool - AR 4.2)
complete 3.2 pool API for DefaultPool
only reap before acquiring connection (when occuped) unless reaper thread is running
support **pool_initial: size** setting with the default pool
README - good to go for 0.1