- Add connection checkin and checkout callbacks to adapter to allow
  adapter-specific customization of behavior (e.g., JRuby w/ JNDI)

- also clean up some cruft remaining from per-thread connection cache

Remove CachedConnectionPerThread per-thread pooling mechanism in favor of a fixed pool with default maximum of 5 connections

- delegating methods left behind

- Also carry allow_concurrency value through to connection adapter
  (for postgresql)

- use new active support Module#synchronize
- allow_concurrency now switches between a null monitor and a
  regular monitor (defaulting to null monitor to avoid overhead)

So far so good, tests still run clean. Next steps: synchronize connection pool access
and modification, and change allow_concurrency to simply switch a real lock for a
null lock.