[SBN read] Moving selecting inputstream from JN in EditlogTailer out of FSNLock

EditlogTailer cost a long time for selecting InputStreams from Journalnode while holding the write lock of FSNLock.
During this period, 8020 handlers of Observer NameNode will be blocked by the FSN Lock.

In theory, selecting inputstreams from JournalNode does not involve changing memory information in NameNode, so we can move the selecting out of the FSN Lock, and it can improve the throughput of Observer NameNode.