update xxhash v2 (coocood#105)

Signed-off-by: Weizhen Wang <[email protected]>

Add a SetAndGet method to return previous value when setting a new one (

coocood#101)

fix evacuate count

add GetWithBuf method to avoid memory allocation

Merge pull request coocood#35 from jb0n/master

fixes for various race conditions

support iterator