Since moving to the new backends, all states (except InmemState) are
Lockers. Add the methods to the State interface to remove a heap of
assertion checks.

All states are lockers, so get rid of extra asertions.

LockWithContext will retry a lock until the context expires or is
cancelled. This will let us implement a `-lock-timeout` flag, and make
use of existing contexts when applicable.

- Have the ui Lock helper use state.LockWithContext.
- Rename the message package to clistate, since that's how it's imported
  everywhere.
- Use a more idiomatic placement of the Context in the LockWithContext
  args.

Add fields required to create an appropriate context for all calls to
clistate.Lock.

Add missing checks for Meta.stateLock, where we would attempt to lock,
even if locking should be skipped.

shouldn't be listed as a common command

Add the -lock-timeout flag to the appropriate commands.
Add the -lock flag to `init` and `import` which were missing it.
Set both stateLock and stateLockTimeout in Meta.flagsSet, and remove the
extra references for clarity.

Backoff the Lock calls exponentially, to a reasonable limit.