Erasure Coding: NameNode may get blocked in waitForLoadingFSImage() when loading editlog

If the editlog contains a transaction for creating an EC file, the NN will get blocked in waitForLoadingFSImage because the following call path:

FSDirectory#addFileForEditLog --> FSDirectory#isInECZone --> FSDirectory#getECSchema --> ECZoneManager#getECSchema --> ECZoneManager#getECZoneInfo --> FSNamesystem#getSchema --> waitForLoadingFSImage

This jira plans to fix this bug and also do some code cleanup.