Refactor consensus output to write to an intermediate store before go…

…ing to the db (#18447)

This PR should have no behavior changes. It introduces a new struct,
`ConsensusCommitOutput`, which stores all writes generated while
processing a consensus commit, prior to writing them to the db.

This is the first stage of caching/quarantining consensus-specific
epoch-db state. The next step will be to hold `ConsensusCommitOutput`
structs in memory until the checkpoints created for the commit have been
certified. This will also require reading from `ConsensusCommitOutput`
(or more likely, a broader caching struct which holds information from a
set of `ConsensusCommitOutput` objects), since required information will
not always be available from the db.