Network.py: Remove unused imports, cleanup imports

Get rid of the evil rename-while-import like "from os import path as os_path"
Remove some imports and vars that weren't being used.
elock: Remove unused "pid" member
filepush: Use a condition instead of restarting a thread on skip

pause/resume is used (exclusively) to be able to skip. The contract
is that nothing is being written after "pause". Instead of stopping
the complete thread, just put the thread into a suspended mode using
a condition/mutex pair on "pause", and release it on "resume".

Starting a thread takes a whole bunch of resources, so needlessly
destroying and creating threads is really bad idea. This should
improve "skip" performance a bit, because the caller no longer needs
to wait for the filepush thread to start.

TODO: Capture the condition/mutex into a C++ class.
Merge remote-tracking branch 'pli/master'