proxy: div by zero

[conradludgate]

Problem

1. In the CacheInvalid state loop, we weren't checking the num_retries. If this managed to get up to 32, the retry_after procedure would compute 2^32 which would overflow to 0 and trigger a div by zero
2. When fixing the above, I started working on a flow diagram for the state machine logic and realised it was more complex than it had to be:
   a. We start in a Cached state
   b. Cached: call connect_once. After the first connect_once error, we always move to the CacheInvalid state, otherwise, we return the connection.
   c. CacheInvalid: we attempt to wake_compute and we either switch to Cached or we retry this step (or we error).
   d. Cached: call connect_once. We either retry this step or we have a connection (or we error) - After num_retries > 1 we never switch back to CacheInvalid.

Summary of changes

1. Insert a num_retries check in the handle_try_wake procedure. Also using floats in the retry_after procedure to prevent the overflow entirely
2. Refactor connect_to_compute to be more linear in design.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

1240 tests run: 1188 passed, 0 failed, 52 skipped (full report)

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