HBASE-25779 HRegionServer#compactSplitThread should be private

Minor refactor. Make the `compactSplitThread` member field of `HRegionServer` private, and gate
all access through the getter method.

Signed-off-by: Yulin Niu <niuyulin@apache.org>
Signed-off-by: Pankaj Kumar <pankajkumar@apache.org>
10 files changed