Add explicit read locks

[keith-turner]

The percolator paper mentioned that read locks were not implemented because they were usually uneeded and expensive. I agree with this, in most situations you do want to read data without a read lock. However I do find that sometimes I would like a read lock. For example consider a use case with graph type data. An edge is added to Fluo and joined with information from the two nodes it connects. I want multiple edge operations to be able to proceed concurrently, as long as the node information does not change. Conceptually, I would like an edge operation to get a read lock on the two nodes its interacting with. If multiple edge operations concurrently get a read lock on the same node, none of them will collide. However a node operation that got a write lock on the node would cause collisions.

From an API perspective, one possible way to implement this is adding the following :

public interface TransactionBase {
  //return snapshot where reads acquire a read lock
  SnapshotBase withReadLock();
}

This would enable writing code like the following :

try(Transaction tx = fluoClient.newTransaction()) {
  //read w/o read lock
  Bytes val1 = tx.get(row1, col1);
  
  //read data with read lock
  Bytes val2 = tx.withReadLock().get(row2, col2);
    .
    .
    .
  tx.commit(); //will collide if row2+col2 was written OR may cause a concurrent write to row2+col2 to fail
}