WL#2912: Add full support for SERIALIZABLE level in NDB

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

Add support for SERIALISABLE level in NDB.
This entails implementing something like next-key locking to be able
to lock ranges from SQL queries. To handle locks of ranges for full table
scan updates can only be done via table lock. No other option exists as
is known.