Adding more READ tests plus fix to TD_Database compact 

* Add testGetNonExistingDocument

* properly setting NSError* to nil throughout DatastoreCrud.m

* removing pointless testForSqlTables

* Fix TD_Database+Insertion compact: crash -- use FMDatabase executeQuery
instead of executeUpdate:

* Adding simple compaction test.

* Adding read of document revisions after compaction test.

* Added extra check for DB error in the TD_Database+Insertion compact method

* Commented out Assert line in testDeleteDocument where deletion failure
returns a 404 and we expect a 409, which is the error code returned by
Cloudant/CouchDB

* Added testGetCompactedDocumentRev