common: Fix includes in massert.h

The massert.h file used errno, but didn't include errno.h.
This commit fixes this issue.
commit: Introduce Timer and Timeout utils

These two classes provide portable interface to a monotonic clock,
which can be used to implement some timeous.
common: Add serialization framework

Add some function which will be used to serialize and deserialize
buffers used in network communication.
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.