NN is unresponsive and loses heartbeats of DNs when Hadoop is configured to use LDAP and LDAP has issues

For Hadoop clusters configured to access directory information by LDAP, the FSNamesystem calls on behave of DFS clients might hang due to LDAP issues (including LDAP access issues caused by networking issues) while holding the single lock of FSNamesystem. That will result in the NN unresponsive and loss of the heartbeats from DNs.

The places LDAP got accessed by FSNamesystem calls are the instantiation of FSPermissionChecker, which could be moved out of the lock scope since the instantiation does not need the FSNamesystem lock. After the move, a DFS client hang will not affect other threads by hogging the single lock. This is especially helpful when we use separate RPC servers for ClientProtocol and DatanodeProtocol since the calls for DatanodeProtocol do not need to access LDAP. So even if DFS clients hang due to LDAP issues, the NN will still be able to process the requests (including heartbeats) from DNs.