Reduce contention in TypeNameIdResolver. Fixes [#2556]

[CodingFabian]

Our production system has shown threads contending on a lock in TypeNameIdResolver:

downstream-7 id=121 state=BLOCKED
    - waiting to lock <0x1a5caa08> (a java.util.HashMap)
     owned by downstream-4 id=118
    at com.fasterxml.jackson.databind.jsontype.impl.TypeNameIdResolver.idFromClass(TypeNameIdResolver.java:95)
    at com.fasterxml.jackson.databind.jsontype.impl.TypeNameIdResolver.idFromValue(TypeNameIdResolver.java:83)
    at com.fasterxml.jackson.databind.jsontype.impl.TypeSerializerBase.idFromValue(TypeSerializerBase.java:82)
    at com.fasterxml.jackson.databind.jsontype.impl.TypeSerializerBase._generateTypeId(TypeSerializerBase.java:67)
    at com.fasterxml.jackson.databind.jsontype.impl.TypeSerializerBase.writeTypePrefix(TypeSerializerBase.java:45)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeWithType(BeanSerializerBase.java:600)
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:147)
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107)
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
    at com.fasterxml.jackson.module.afterburner.ser.ObjectMethodPropertyWriter.serializeAsField(ObjectMethodPropertyWriter.java:40)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:400)
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1392)
    at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1120)
    at com.fasterxml.jackson.databind.ObjectWriter.writeValueAsBytes(ObjectWriter.java:1017)

This was reported in #2566. Investigation and benchmarks have shown that the contention is actually on the get/ read of the _typeToId map, because it is synchronized.

Moving the write part out has no measurable effect since the map is populated.

Swapping for a ConcurrentHashMap gives around 10% additional throughput on heavy json serialisation (using the same object mapper. using different object mappers uses different caches).

For symmetry, _idToType has also been changed. Technically the previous version was not threadsafe, as even concurrent HashMap reads can deadlock.

[cowtowncoder]

Ok sounds good. Will merge and tweak slightly: will be in 2.10.2 (hoping there isn't any potential regression... famous last words).