Migrate to `NIOLock`

[Colgates]

Migrates usages of Lock to NIOLock to fix deprecation warnings

[0xTim]

@Colgates looks like there are some build failures in the tests with 5.4

[codecov-commenter]

Codecov Report

Merging #95 (de26df3) into main (c3329e4) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head de26df3 differs from pull request most recent head abed7ca. Consider uploading reports for the commit abed7ca to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #95 +/- ## ======================================= Coverage 95.82% 95.82% ======================================= Files 19 19 Lines 1199 1199 ======================================= Hits 1149 1149 Misses 50 50 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `95.82% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/vapor/async-kit/pull/95?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor) | Coverage Δ | | |---|---|---| | [.../ConnectionPool/EventLoopGroupConnectionPool.swift](https://codecov.io/gh/vapor/async-kit/pull/95/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor#diff-U291cmNlcy9Bc3luY0tpdC9Db25uZWN0aW9uUG9vbC9FdmVudExvb3BHcm91cENvbm5lY3Rpb25Qb29sLnN3aWZ0) | `94.77% <100.00%> (ø)` | |

[gwynne]

The fixes from this PR are now included verbatim in #96 alongside a couple of additional cleanups, with authorship information duly noted; I'm going to close this PR out accordingly - thanks again for your work on this!