[proxy] Fix dynamic rate limiter

[khanova]

Problem

There was a bug in dynamic rate limiter, which exhausted CPU in proxy and proxy wasn't able to accept any connections.

Summary of changes

1. if self.available > 1 -> if self.available >= 1
2. remove timeout_at to use just timeout
3. remove potential infinite loops which can exhaust CPUs.

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

[sharnoff]

Left some comments - not familiar with the code, feel free to adopt / ignore as needed.

Approving to avoid timezone blockage.

[github-actions]

3174 tests run: 3035 passed, 0 failed, 139 skipped (full report)

---

Flaky tests (1)

Postgres 16

• test_subscriber_restart: release

Code coverage* (full report)

• functions: 31.4% (6558 of 20860 functions)
• lines: 48.4% (50803 of 104958 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
e9f699a at 2024-06-04T01:14:29.353Z :recycle:}