Add RegionObserver pre hooks that operate under row lock

The coprocessor hooks were placed outside of row locks. This was meant to sidestep performance issues arising from significant work done within hook invocations. However as the security code increases in sophistication we are now running into concurrency issues trying to use them as a result of that early decision. Since the initial introduction of coprocessor upcalls there has been some significant refactoring done around them and concurrency control in core has become more complex. This is potentially an issue for many coprocessor users.

We should do either:

• Move all existing RegionObserver pre* hooks to execute under row lock.

• Introduce a new set of RegionObserver pre* hooks that execute under row lock, named to indicate such.

The second option is less likely to lead to surprises.

All RegionObserver hook Javadoc should be updated with advice to the coprocessor implementor not to take their own row locks in the hook. If the current thread happens to already have a row lock and they try to take a lock on another row, there is a deadlock risk.

As always a drawback of adding hooks is the potential for performance impact. We should benchmark the impact and decide if the second option above is a viable choice or if the first option is required.

Finally, we should introduce a higher level interface for managing the registration of 'user' code for execution from the low level hooks. I filed HBASE-11125 to discuss this further.