[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.
made some changes suggested in review
get rid of some unnecessary preprocessor usage

We are modern programmers with modern tools
Merge pull request #43 from amtep/update

[performance] Speed up reading from USB