Fix panic when Subscriber is closed during explicit Sync

[gammazero]

If the Subscriber is closed during an explicit Sync, items used by the explicit sync may become invalid. If the events channel is closed and the explicit sync tries to write to it, the a panic will happen.

This is fixed by using a sync.WaitGroup to track the explicit syncs in progress and a synchronized variable to prevent any additional explicit Sync calls during close.

[codecov-commenter]

Codecov Report

Patch coverage: 75.00% and project coverage change: +0.09 🎉

Comparison is base (9e14275) 52.50% compared to head (7b78f32) 52.60%.

❗ Current head 7b78f32 differs from pull request most recent head 32188c4. Consider uploading reports for the commit 32188c4 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   52.50%   52.60%   +0.09%     
==========================================
  Files          59       59              
  Lines        4706     4722      +16     
==========================================
+ Hits         2471     2484      +13     
- Misses       1967     1969       +2     
- Partials      268      269       +1     

Impacted Files	Coverage Δ
dagsync/subscriber.go	76.53% <75.00%> (-0.05%)	⬇️

... and 1 file with indirect coverage changes

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[github-actions]

Suggested version: v0.2.1

Comparing to: v0.2.0 (diff)

Changes in go.mod file(s):

(empty)

gorelease says:

# diagnostics
required module github.com/microcosm-cc/bluemonday@v1.0.1 retracted by module author: Retract older versions as only latest is to be depended upon

# summary
Suggested version: v0.2.1

gocompat says:

Your branch is up to date with 'origin/main'.

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