hotfix/fix use-after-free caused by cancelled service tracker.

[PengZheng]

This PR fixes #595, a use-after-free introduced by a previous attempt to avoid deadlock, by splitting the creation of a service tracker into two steps:

1. Create a closed service tracker if not cancelled under the protection of ctx->mutex
2. Open the tracker without holding ctx->mutex (no risk of deadlock).

[codecov-commenter]

Codecov Report

Merging #596 (e5a0a37) into master (456de19) will decrease coverage by 0.48%.
The diff coverage is 80.76%.

❗ Current head e5a0a37 differs from pull request most recent head 7c65bfc. Consider uploading reports for the commit 7c65bfc to get more accurate results

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
- Coverage   79.00%   78.52%   -0.48%     
==========================================
  Files         234      234              
  Lines       35386    35389       +3     
==========================================
- Hits        27955    27790     -165     
- Misses       7431     7599     +168     

Files Changed	Coverage Δ
libs/framework/src/service_tracker.c	77.70% <75.00%> (+0.07%)	⬆️
libs/framework/src/bundle_context.c	80.53% <88.88%> (+0.17%)	⬆️
...iscovery_zeroconf/src/discovery_zeroconf_watcher.c	88.51% <100.00%> (-0.04%)	⬇️

... and 6 files with indirect coverage changes

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[pnoltes]

LGTM