Heavily revise the implementation of EventLoopConnectionPool

[gwynne]

Overview

This is a temporary stopgap measure to clean up some of the more extant issues with the existing implementation, pending the integration of a much more robust and advanced connection pool solution.

While technically there is no change in the public API surface, this is nonetheless being marked as semver-minor due to the significant changes in behavior. This PR has already revealed a query ordering bug in the Fluent benchmarks just by behaving more correctly.

This should hopefully fix the issue described in #101, but I haven't been able to locally reproduce the race condition despite being fairly sure as to its cause, so I can't verify.

More details

• Now uses Deque instead of the obsolete CircularBuffer for tracking available connections
• Now uses an OrderedDictionary and numeric waiter IDs to track waitlists rather than the obsolete CircularBuffer
• Checking for on-EventLoop execution is now handled more idomatically
• Queries are now always served in order instead of new queries getting priority over those already on the waitlist when a new connection becomes available
• Removing completed waiters from the waitlist and cancelling their timeouts is now considerably more reliable
• If more than one connection has become available when a connection is released, multiple waiters are now serviced immediately
• Waiters that are still waiting during pool shutdown no longer emit spurious request timeout logs
• The time taken to establish a new database connection (if needed) now counts against a waiter's timeout duration (making the duration much more accurate)

[github-advanced-security[bot]]

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[codecov-commenter]

Codecov Report

Merging #102 (deda984) into main (4ee8c97) will decrease coverage by 0.07%. Report is 1 commits behind head on main. The diff coverage is 95.78%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #102 +/- ## ========================================== - Coverage 95.70% 95.63% -0.07% ========================================== Files 19 19 Lines 955 963 +8 ========================================== + Hits 914 921 +7 - Misses 41 42 +1 ``` | [Files Changed](https://app.codecov.io/gh/vapor/async-kit/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [...ncKit/ConnectionPool/EventLoopConnectionPool.swift](https://app.codecov.io/gh/vapor/async-kit/pull/102?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9Db25uZWN0aW9uUG9vbC9FdmVudExvb3BDb25uZWN0aW9uUG9vbC5zd2lmdA==) | `94.85% <95.78%> (-0.46%)` | :arrow_down: |

[gwynne]

Holding off on merging this until @0xTim can glance at it too, given how fundamental this awful code is to Fluent.

[penny-for-vapor[bot]]

These changes are now available in 1.18.0