SWIFT-749 use ELG provider, update shutdown API

[kmahar]

This PR updates our client initializer to accept an EventLoopGroupProvider rather than an EventLoopGroup.
As a result I needed to update our shutdown API since it can no longer return a future.

I took a lot of inspiration writing shutdown code from:

• NIOThreadPool
• MultiThreadedEventLoopGroup
• AsyncHTTPClient

Comments inline

[kmahar]

also all this definitely necessitates updating guides/examples but I figured I'd see what you all have to say on the API before I do that

[patrickfreed]

API looks good, great job on researching what the current standards are. I just a have a few comments about the internals.

[codecov-io]

Codecov Report

Merging #446 into master will increase coverage by 0.12%.
The diff coverage is 90.44%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   76.59%   76.72%   +0.12%     
==========================================
  Files         117      117              
  Lines       12984    13056      +72     
==========================================
+ Hits         9945    10017      +72     
  Misses       3039     3039              

Impacted Files	Coverage Δ
Sources/MongoSwift/ConnectionPool.swift	59.47% <83.56%> (+12.25%)	⬆️
Sources/MongoSwift/MongoClient.swift	86.75% <93.75%> (+0.94%)	⬆️
Sources/MongoSwift/APM.swift	83.54% <100.00%> (+<0.01%)	⬆️
Sources/MongoSwift/Operations/Operation.swift	96.87% <100.00%> (+1.10%)	⬆️
Sources/MongoSwiftSync/MongoClient.swift	54.41% <100.00%> (ø)
Tests/MongoSwiftTests/AsyncTestUtils.swift	67.24% <100.00%> (+3.44%)	⬆️
Tests/MongoSwiftTests/MongoClientTests.swift	98.97% <100.00%> (+0.61%)	⬆️

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[kmahar]

alright I've made significant changes based on our conversations, will make some comments inline. the gist is that the client now only tracks if it has been closed, which is just for the purpose of asserting in deinit. otherwise we let the connection pool and NIOThreadPool handle erroring.

[nbbeeken]

Just some nits that are optional so lgtm 🚀