[core] optimize template rendering

[cyrilou242]

Before:

Benchmark                                  Mode  Cnt     Score    Error  Units
StringTemplateUtilsBenchmark.applyContext  avgt    7  1480.610 ± 56.571  us/op

after:

Benchmark                                  Mode  Cnt   Score   Error  Units
StringTemplateUtilsBenchmark.applyContext  avgt    7  55.539 ± 0.272  us/op

Problem: StringTemplateUtils#applyContext - the method to apply properties to template - was extremely slow.
An ObjectMapper was instantiated at each call, so the ser/deserializer obtained by reflection (slow) were re-computed each time.
Optimization: re-use the ObjectMapper. It's a bit tricky because we use custom ser/deser to inject values. Values change at each call.
So we mutate the custom ser/deserializer of the re-used ObjectMapper at each call. This is not thread safe so we introduce a pool of re-usable ObjectMappers.

For an alert with 1000 enumeration items:

• before: applying properties takes ~ 1000 *1480us ~= 1.5 seconds --> feels to slow in the UI
• after: applying properties takes ~ 1000 *55us ~= 50 milliseconds