Implemented ydb_istream module.

Extracts the inserted tuples.
Implemented ydb_dstream module.

Extracts the deleted tuples.
Changed to send a separate message for each diff.
Stubbed out ydb_rstream module.

Defined helper functions for inserting and deleting tuples from the
current representation of the relation.
Implemented logic for processing received diffs.

Extracts the PLUS and MINUS tuples separately and calls do_insert/2
and do_delete/2 on them.

Fixed an issue with do_insert/2 where was not catching the badarg
exception correctly.

Fixed an issue with get_relation/1 where was not constructing a multi-
set based on the count of tuple in the synopsis.
Merge branch 'rel_to_stream'