new local lru cache

[binlijin]

When we set Historical cache to the flowing Configuration

druid.cache.type=local
druid.cache.sizeInBytes=2000000000 //almost 2G

we find the historical have 5.6G of cache data.

And when we change to

druid.cache.type=local
druid.cache.sizeInBytes=500000000 //almost 500M

we find the historical have 2G of cache data.

Historical cache data is larger than the configured druid.cache.sizeInBytes.

[drcrallen]

is there any guard to prevent size from being 0?

[drcrallen]

This will need to be reconciled with #1849

[drcrallen]

Can this default to the number of processing threads?

[xvrl]

@binlijin did you look at what I did in #1849 before working on this?
My experience is that guava cache might have worse latencies, that's why I haven't been gung ho on getting #1849 merged.

[drcrallen]

Also note #1836

There seem to be a few outstanding issues with the caching done currently that are not called out here:

1. Latency of guava cache
2. Benchmarks on concurrent access
3. Improvements (or no change) on behavior of large single-scans clobbering the cache.

[binlijin]

@drcrallen @xvrl thanks, i will have a look at them，and others should hold on this.