[CL Incentives] Remove ability to add to frozen positions

[AlpinYukseloglu]

Background

Now that we key positions by joinTime and freezeDuration, it is no longer possible for people to add to an existing position. While we might still want to special-case non-frozen positions to save state, we should at the very least remove unnecessary code complexity from updates for uptime accumulators.

Suggested Design

• initOrUpdatePositionUptime -> initPositionUptime
• Return error if record exists and last pool update is Before current blocktime
  • This is to ensure that users can create multiple txs in the same block (although not sure why we wouldn't just error here)

Acceptance Criteria

• New and existing tests pass

[stackman27]

Completed ##4508