Add performance counters.

This commit adds performance counters for each transaction which are rolled up to the database level on each commit/rollback. Counters are meant to be a very fast way to track what is going on in the database. A few timers are also added in areas where the time.Now() overhead is not noticeable.

The DB.Stat() function is now deprecated since the `bolt` CLI performs similar functions with better granularity. The DB.Check() is also available for checking consistency.

Running `go test -v -stats` will now print out the performance counters for each DB used in the tests.

Fixes #108.

/cc @snormore