Security update: Memtable hardening

This releases addresses a few concerns in the memtable:

• Inconsistent lock-free reads on mutable memtables
• Now copies key and values at time of putting data into a memtable instead of requiring the call site to do it
• Memtable Iterator now supports reading at a specific sequence number
• Fixes atomic/non-atomic mixing in sequence numbers of memtables

This is a maintenance release, it's recommended people upgrade.