[storage] remove filePath argument from StoragePlugin::eventGenerated()

Not used anywhere.
[storage] discard object's all cached properties on ObjectInfoChanged

For example on handleFSModify() file size wasn't removed from the cache
and thus old value was kept being reporter to the initiator.
Merge pull request #44 from xhaakon/update

Discard object's all cached properties on ObjectInfoChanged.
[performance] Speed up reading from USB

This patch allows the BulkReaderThread and the MTP responder to work
at the same time, instead of taking turns blocking.

It gives about a 20% speed improvement on Jolla hardware.
[debug] disable writing to storage backend, for perf measurement