Add a method on FDBRecordStore to check if a record exists

[alecgrieser]

Currently, the only way to see if a record exists is to try and load the record. The FDB client doesn't really allow us to check for the presence of keys (at the moment), so the best we can do is something like read the record from the database but not deserialize it (and save some CPU work). If, at some point, the FDB client allows us to query for the presence of keys, we could theoretically change that function to save some network bandwidth as well.