GEOMESA-387 Adding synchronization to createSchema method in AccumuloDataStore

assuming this is one data store shared between multiple threads...wouldn't a simply ```try/finally``` with a reentrant lock or ```this.synchronized``` be simpler:

```
lock.lock()
try {
  //create it
} finally {
  lock.unlock()
}
````
or simpler with a mutex lock:
```
this.synchronized {
   // foobar
  }
```

Since we really shouldn't be doing this much we don't expect lots of threads to be concurrently calling it, so we aren't worried about performance and are going for simplicity. Also, we shouldn't care about ordering creates for multiple schema types per datastore.

The other question is are we sure that there aren't multiple AccumuloDataStores instances doing this (or multiple JVMs)