Interrupt cache prefetch thread when a heap usage threshold is reached

This change adds a new configuration property, "hbase.rs.prefetchheapusage", which specifies a heap usage limit for the prefetch threads to be allowed to execute. This is to prevent an overhead heap usage by bucket cache mapping objects when using file or offheap engines. This value should be defined as a double representing the heap usage percentage, and it's default set to 1d (100% heap), disabling this feature. For example, to set an 80% heap usage threshold, set "hbase.rs.prefetchheapusage" to "0.8" in the RegionServer hbase-site.xml configuration.