Fix bug #330 (Calling FileIndexManager.getFileInfoList() while another such
call is already in progress fails) - keep track of existing index-sync
promise so we can just wait on it instead of failing.
Merge pull request #3064 from adobe/pflynn/fileindexmanager-reentrancy

Fix bug #330 (Calling FileIndexManager.getFileInfoList() fails if another such call in progress)