Add way to avoid creating duplicate FATE operation

[keith-turner]

For automatic splits the following happens.

• Tablet group watcher periodically scans the metadata table looking for tablets to split
• When a tablet is found to split, the manager tracks in memory that the tablet is in the process of splitting
• A Fate operation is started to split the tablet.

In #4178 the above tracking that is done in manager memory is moved to FATE. However the FATE implementation is not complete, its only stubbed out in #4178. To complete #4178, need the following capabilities in FATE.

• The ability to specify a key when creating a FATE operation. For the split this case this key would be the extent. The fate operation should only be created if the key does not exists.
• The ability to handle failures in the case where the FATE operation is created with a key, but is not seeded.

The following is an example of one way this could work.

FateStore store = ...;
KeyExtent extent = ...; // extent to split
OptionalLong txid = store.create("split", toByteArray(extent));
if(!txid.isPresent()){
   // check to see if it was created but not seeded
    txid = store.findByKey("split", toByteArray(extent));
     if(txid.isPresent()){
            // TODO reserve and if in the new state, then seed fate op
      }
} else {
   // TODO seed the fate op
}

Maybe the above code would be more efficient as something like Optional<Pair<Long, TStatus>> createOrFind(key).

The implementation of the create(key) function could hash the key to obtain a 64 bit fate id. It would need to handle collisions in a similar way to a hashmap.