fail. This fixes it by sorting out the auto increment identifier

There was code in the GeneAdaptor which could be handled by the DBEntryAdaptor
and would be more reusable by others if it was there. Also added a test
case for the new method get_distinct_external_dbs() in DBEntryAdaptor.

when inserting 3 entries in parallel, should only store once
but depending on how fast mysql catches up, the return dbID corresponds to the first or the last attempt
now allowing for this change of mind

Trying to improve the current test cases. Accidentally caused some issues from rerunning the code multiple times as well

Patching issue with there being a difference between NULL and and empty string WRT description and also plugged a gap in the code when we were trying to query using a display_label which the api allows to be NULL but the DB does not

	QUERY_IDENTITY	->	XREF_IDENTITY
	TARGET_IDENTITY	->	ENSEMBL_IDENTITY
	QUERY_START	->	XREF_START
	QUERY_END	->	XREF_END
	TRANSLATION_START	->	ENSEMBL_START
	TRANSLATION_END	->	ENSEMBL_END

tested 2 new methods: to get db_name for an external_db_id and fetch db_entries for a source (db_name)