Change LRUMap to just evict one entry when maxEntries reached

[pjfanning]

Current implementation clears full map when maxEntries reached. jackson-dataformat-csv has its own separate LRUMap[1] and it only evicts the eldest entry when you add a new entry after the maxEntries is reached. Would it be possible to have jackson-databind LRUMap use similar logic? The change would require LRUMap to subclass LinkedHashMap.

[1] https://github.com/FasterXML/jackson-dataformats-text/blob/2.14/csv/src/main/java/com/fasterxml/jackson/dataformat/csv/impl/LRUMap.java

[pjfanning]

Closing as this rehashes #2502 and FasterXML/jackson-module-scala#428. Making the change in the issue description would incolve dropping the ConcurrentHashMap that currently backs LRUMap.

[cowtowncoder]

Makes sense. And just to be clear: the reason LRUMap just cleared it all was because incremental updates were not available using backing map choice (originally LinkedHashMap was used, but there were concurrency problems as both reads and writes need synchronization and only writes were sync'ed... long story). Clear-all was not a feature we want, just the limitation of the approach.

[cowtowncoder]

Hooray! Big thanks to @pjfanning for contributing the implementation and @ben-manes for helping with suggestions.