Recycle via reference counting

[felixbarny]

• Recycling is not done explicitly but implicitly when the reference count goes to 0
• This also simplifies async context propagation
  • There's no need to copy the context to the other thread
  • The Span is just activated on the other thread
    If it's ended before the child starts it is still not recycled because the reference count is not at zero yet.
• This is in also in preparation for breakdown metrics
  • You need to have a reference to the parent in order to calculate the self-time of spans
  • In order to be able to calculate the self-time in async scenarios, it's required to activate the span as opposed to just it's TraceContext

[codecov-io]

Codecov Report

Merging #587 into master will decrease coverage by 0.36%.
The diff coverage is 64.1%.

@@             Coverage Diff              @@
##             master     #587      +/-   ##
============================================
- Coverage      63.7%   63.34%   -0.37%     
  Complexity       68       68              
============================================
  Files           189      189              
  Lines          7431     7428       -3     
  Branches        871      861      -10     
============================================
- Hits           4734     4705      -29     
- Misses         2433     2444      +11     
- Partials        264      279      +15

Impacted Files	Coverage Δ	Complexity Δ
...m/agent/plugin/api/TransactionInstrumentation.java	52.38% <ø> (+7.14%)	0 <0> (ø)	⬇️
...agent/configuration/validation/RegexValidator.java	100% <ø> (ø)	0 <0> (ø)	⬇️
.../co/elastic/apm/agent/impl/payload/SystemInfo.java	79.61% <ø> (ø)	0 <0> (ø)	⬇️
...pm/agent/objectpool/impl/QueueBasedObjectPool.java	39.39% <ø> (ø)	0 <0> (ø)	⬇️
...c/apm/agent/impl/async/SpanInScopeBaseWrapper.java	62.96% <ø> (+4.62%)	0 <0> (ø)	⬇️
.../v6_4/ElasticsearchClientAsyncInstrumentation.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...pm/agent/configuration/converter/TimeDuration.java	90% <ø> (ø)	0 <0> (ø)	⬇️
...m/agent/objectpool/impl/ThreadLocalObjectPool.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...n/java/co/elastic/apm/agent/util/VersionUtils.java	60% <ø> (ø)	0 <0> (ø)	⬇️
...elastic/apm/agent/impl/payload/ProcessFactory.java	79.59% <ø> (ø)	0 <0> (ø)	⬇️
... and 199 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 17f6985...6075195. Read the comment docs.

[eyalkoren]

I know this is a big PR already, but can we also lose TraceContextHolder and restore the simpler Span hierarchy now, or do we still need it?