Fix memory leak due to use of ConcurrentBag

[alistairjevans]

Ensure we empty the _registrations bag in DefaultRegisteredServicesTracker when we dispose the tracker; this ensures any ThreadLocal references are cleared up and memory is not leaked.

Fixes #1252

[codecov]

Codecov Report

Merging #1257 (937f0ab) into develop (3ca27f8) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1257      +/-   ##
===========================================
+ Coverage    76.34%   76.38%   +0.03%     
===========================================
  Files          185      187       +2     
  Lines         4870     5085     +215     
  Branches      1004     1034      +30     
===========================================
+ Hits          3718     3884     +166     
- Misses         679      707      +28     
- Partials       473      494      +21     

Impacted Files	Coverage Δ
...e/Registration/DefaultRegisteredServicesTracker.cs	83.84% <100.00%> (+0.12%)	⬆️
src/Autofac/Features/Metadata/Meta{T,TMetadata}.cs	100.00% <0.00%> (ø)
...ilder{TLimit,TActivatorData,TRegistrationStyle}.cs	76.44% <0.00%> (ø)

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[tillig]

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