3.x: Add ReadWriteLock for each table's tablet set

[Bouncheck]

Each table now has a dedicated lock for its set of tablets.
Multiple sets can be modified concurrently. Most importantly without
copying the whole set.
Should improve times, mostly in cases of large amount of tablets and
reduce number of misses during tablet shuffling.

[Bouncheck]

I was hoping that previous implementation was causing the imbalance after adding nodes but testing seems to indicate that this change had no impact on that.
Still, I think there's no reason not to merge this anyway.

[dkropachev]

I was hoping that previous implementation was causing the imbalance after adding nodes but testing seems to indicate that this change had no impact on that. Still, I think there's no reason not to merge this anyway.

Pervios implementation caused parallel read/write conflicts, you are right, let's merge it.