Created a new WALKey Interface and a WALKeyImpl. The WALKey Interface
is surfaced to Coprocessors and throughout most of the code base.
WALKeyImpl is used internally by WAL and by Replication which need
access to WALKey setters.
Methods that were deprecated in WALObserver because they were exposing
Private audience Classes have been undeprecated now we have WALKey.
Moved over to use SequenceId#getSequenceId throughout. Changed
SequenceId#getSequenceId removing the IOE.
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