Change FuseAppLoop so that it can process messages asynchronously.

Previously FuseAppLoopCallback needs to return values in a synchrnous
manner. The CL changes it to asynchronous mannger so that apps can
process FUSE message asynchrnously.

Bug: 35229514
Test: FuseAppLoopTest
Change-Id: I8edcfdb003a25cfd5e9c490ec871140220b21e35
3 files changed