perf(orca): reuse ObjectMapper instances

[mattnworb]

creating a new ObjectMapper instance for each Stage or other per-request
contexts can be wasteful; ObjectMapper is safe to reuse across threads
(after it is configured). See spinnaker/spinnaker#4367 for more details.

[jtk54]

LGTM.

[ezimanyi]

This seems reasonable to me, but would like @robzienert or @robfletcher to take a look as I know that there have been some subtle bugs with objectMapper in the past.

[robfletcher]

Are there any places where the ObjectMapper is customized for particular use cases? Just wondering why we had it as a factory in the first place.

[robzienert]

This should be fine. 99... 95... 96.7% sure.

[mattnworb]

@robzienert I did find a few places in the Orca codebase where the new ObjectMapper instance returned by OrcaObjectMapper.newInstance() is mutated / configured in different ways:

orca/orca-integrations-gremlin/src/main/java/com/netflix/spinnaker/orca/config/GremlinConfiguration.kt

Lines 41 to 45 in cd6b6ca

	val mapper = OrcaObjectMapper
	.newInstance()
	.setPropertyNamingStrategy(
	PropertyNamingStrategy.SNAKE_CASE)
	.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS) // we want Instant serialized as ISO string

orca/orca-keel/src/main/kotlin/com/netflix/spinnaker/orca/config/KeelConfiguration.kt

Lines 62 to 66 in cd6b6ca

	@Bean fun keelObjectMapper() =
	OrcaObjectMapper.newInstance()
	.registerModule(KotlinModule())
	.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
	}

orca/orca-kayenta/src/main/kotlin/com/netflix/spinnaker/orca/kayenta/config/KayentaConfiguration.kt

Lines 67 to 69 in 2b9de27

	val mapper = OrcaObjectMapper
	.newInstance()
	.disable(WRITE_DATES_AS_TIMESTAMPS) // we want Instant serialized as ISO string

I left all of those unchanged in this PR, since from what I could tell, those call sites were not creating one ObjectMapper per request / unit of work etc. (and plus they seem to have legitimate reasons to configure the ObjectMapper instance differently than other regions of code).