Mutex cleanup

@sue, would you please review this one?

I moved the serialization lock from WT_CONNECTION into WT_BTREE, the btree code is the only user (and it might reduce contention when we have a workload with lots of files being modified, for example, LSM).

And, if we're using pthread locking for spinlocks, lock initialization can fail and we don't want to destroy uninitialized memory, track if the lock was ever initialized.

Thank you!