All scan requests should be handled by scan handler threads in RWQueueRpcExecutor

When we use RWQueueRpcExecutor, all scans should be handled by scan handler threads, not read handler.

Before HBASE-17508, when calling openScanner() in client, a region server doesn't make results, just open scanner and return scanner id. So, this request(open) is executed in read handlers intentionally.
 

However, since HBASE-17508,, actual scan behavior happened while opening a scanner,

I think this request should probably be executed in scan handlers when using RWQueueRpcExecutor.