BatchSpanProcessor optimizations - Separate control signal queue, and wake up background thread only when required.

[lalitb]

Fixes: Towards #2493

Changes

Similar to the BatchLogProcessor in #2494 :

• wake up the background thread ONLY when needed
• Separate control signal queue ( to ensure that the shutdown signal is not lost when message queue is full).

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 90.51095% with 13 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (78db32c) to head (6d2edf0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/trace/span_processor.rs	90.5%	13 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #2526    +/-   ##
======================================
  Coverage   77.9%   77.9%            
======================================
  Files        123     123            
  Lines      22944   23056   +112     
======================================
+ Hits       17880   17980   +100     
- Misses      5064    5076    +12     

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[cijothomas]

this reset need to happen before the exporting. Just like logs.

[lalitb]

Good point. Updated accordingly.

[cijothomas]

LGTM, need to make this fix: https://github.com/open-telemetry/opentelemetry-rust/pull/2526/files#r1924235350 before merge