Replace Jackson with DSL-JSON

[felixbarny]

Benchmarking shows that serializing payloads with dsl-json is x2.4 faster and requires -86% less allocations.

DSL-JSON is a nice library which focuses on fast and garbage free JSON serialization. It only has a small byte[] buffer, which is reused and otherwise directly writes into a OutputStream, so there are no temporary objects created. It also has a mode where it can automatically serialize annotated POJOs. I opted for a more manual approach to have more control about the resulting JSON (see hasContent() checks and explicitly serializing TransactionId with UUIDConverter.serialize(id.getMostSignificantBits(), id.getLeastSignificantBits(), jw);)

My recent additions to dsl-json ngs-doo/dsl-json#48 and ngs-doo/dsl-json#49 make it possible to also serialize StringBuilders and transaction ids in the UUID format in a garbage free way. The only thing which requires allocations now is creating a ISO date formatted string for the Transaction.timestamp and creating Map.entrySet().iterator()s for iterating over custom Context.tags. I'm not quite sure why JIT can't replace the iterator heap allocations with stack allocations, but maybe JMH's GC profiler also reports stack allocations in gc.alloc.rate.norm. Regarding the timestamp serialization, there is an open issue about adding support for epoch timestamps: elastic/apm-server#850. That would make the JSON serialization essentially completely garbage free.

As an additional benefit, the total size of the agent jar drops from 2.8 to 1.2 mb as the replaced Jackson library is much heavier.

This PR is currently blocked until dsl-json 1.7.4 is released.

[codecov-io]

Codecov Report

Merging #69 into master will increase coverage by 1.25%.
The diff coverage is 81.16%.

@@             Coverage Diff              @@
##             master      #69      +/-   ##
============================================
+ Coverage     75.29%   76.55%   +1.25%     
- Complexity      547      637      +90     
============================================
  Files            71       71              
  Lines          1919     2354     +435     
  Branches        146      187      +41     
============================================
+ Hits           1445     1802     +357     
- Misses          397      448      +51     
- Partials         77      104      +27

Impacted Files	Coverage Δ	Complexity Δ
...co/elastic/apm/impl/transaction/TransactionId.java	78.78% <ø> (ø)	15 <0> (ø)	⬇️
.../java/co/elastic/apm/impl/error/ExceptionInfo.java	58.82% <ø> (ø)	7 <0> (ø)	⬇️
...ain/java/co/elastic/apm/impl/ElasticApmTracer.java	79.2% <ø> (ø)	33 <0> (ø)	⬇️
...n/java/co/elastic/apm/impl/error/ErrorCapture.java	60.86% <ø> (ø)	8 <0> (ø)	⬇️
...a/co/elastic/apm/impl/transaction/Transaction.java	71.91% <ø> (+1.12%)	26 <0> (-1)	⬇️
...o/elastic/apm/impl/error/TransactionReference.java	71.42% <ø> (ø)	3 <0> (ø)	⬇️
...main/java/co/elastic/apm/impl/payload/Service.java	90.9% <ø> (ø)	14 <0> (ø)	⬇️
...c/apm/impl/stacktrace/StacktraceConfiguration.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...e/src/main/java/co/elastic/apm/impl/error/Log.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...n/java/co/elastic/apm/impl/transaction/SpanId.java	77.77% <ø> (ø)	11 <0> (ø)	⬇️
... and 32 more

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