Adding an InitTablet feature to vttablet.

It only covers a subset of what the real InitTablet does,
but should be enough to not have to initialize tablets before starting them.
@enisoc @aaijazi 
Isn't agent.LockTimeout supposed to be the timeout per lock/unlock action? This block is currently using it as the overall timeout for the whole "add a cell to a shard" operation, since the same ctx is used for everything.