KAFKA-14711: Add new StateStore interfaces

[nicktelford]

These are the new interfaces detailed in KIP-1035: "StateStore managed changelog offsets".

This PR introduces the interfaces changes, but makes otherwise no consequential behavioural changes.

Outside of StateStore.java, all changes are essentially just a rename of all invocations of StateStore#flush to instead call StateStore#commit.

The changelogOffsets being passed in these invocations is currently unused: the behaviour of StateStore#commit remains identical to StateStore#flush before these changes.

A new implementation of StateStore#commit that actually uses these offsets, along with changes to the use-site (in ProcessorStateManager and GlobalStateManager) will come in a later PR.

Many strings, including documentation, and some variable names, have also been renamed (from "flush" to "commit"), to maintain consistency with the method they relate to.

One exception is the flush-rate metric, which has not been renamed, because it will instead be deprecated in favour of a new commit-rate metric, which will be introduced in another PR.

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The only change in behaviour is as follows: calling StateStore#flush from within a Processor is now a guaranteed no-op.

In the future, this will throw an UnsupportedOperationException, but to ensure no changes to end-user experience, we currently make it a no-op.

Previously, any call to StateStore#flush from a Processor would have made no difference to program semantics, but likely would introduce performance problems for RocksDB. This is because it would force a flush of RocksDB memtables to disk on every invocation, which if naively used could be on every Record.

Consequently, making this a no-op should not make a difference for end-users, except potentially improving performance if they were incorrectly calling this method.

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[github-actions[bot]]

A label of 'needs-attention' was automatically added to this PR in order to raise the attention of the committers. Once this issue has been triaged, the triage label should be removed to prevent this automation from happening again.