Use path relative to src/ when inluding headers from common

This commit changes all the source files to use paths relative to
src/ directory when including files from the src/common directory.
This makes it more clear which files from a serie of #includes
belong to which module.
common: Fix includes in massert.h

The massert.h file used errno, but didn't include errno.h.
This commit fixes this issue.
common: Add time utilities

Add facilities for portable access to monotonic clocks, time measurement
and timeout management.
common: Add framework for handling packets

Add code that handles incomming and outgoing packets. Packets that are
handled by a new code have common header consisting of a packet number
and packet content length. Content of all new packets starts with 4 byte
version.
common: Add NetworkAddress class

This commit introduces a class, which represents an address
of a server over a network. Currently this class is implemented
as a pair (ipv4, port).
mount: Refactor write worker

This commit refarors write_worker function in mount. It does not
change the behavior in any way, just makes the code easier to
understand for future developers.