- 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)

- Distinguis meaning of "active_connections" to always mean connections
  associated with the current thread

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.

Signed-off-by: NMichael Koziarski <michael@koziarski.com>

Signed-off-by: NJoshua Peek <josh@joshpeek.com>

Signed-off-by: NPratik Naik <pratiknaik@gmail.com>

[#501 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#831 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#348 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#302 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#798 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

* Set the association target on assignment;
* Reset target to nil on reset, rather than empty array.
Signed-off-by: NMichael Koziarski <michael@koziarski.com>
[#895 state:committed]

Back to fetching all versions in ruby instead of letting SQL do it as it's difficult to get all databases to convert the text value to a number with the same SQL
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#905 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Use DECIMAL instead of INTEGER when casting as mysql doesn't work with just "INTEGER" and other databases don't like "UNSIGNED" which mysql requires

And don't mask exceptions.
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Cache migrated versions list in Migrator and use it to fetch the latest migrated version name [#845 state:resolved]

Also optimized Migrator#current_version class method to fetch
only the latest version number and not all of them.

With this change no matter how many migrations there are the
schema_migrations table is only SELECTed from once.
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

This matters when the has_one is defined with an order in which case
there is an expectation that the first one will be loaded.

[#904 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>