Ensure the timestamp always increases in zero-copy libdlog

Ensure the timestamp always increases in zero-copy libdlog

The kernel guarantees that it won't decrease, but if function is
executed in quick repetition it might happen that we get the same value
twice in a row. This is unacceptable, since the timestamp is the only
thing we rely on in order to sort the messages.

This mirrors a similar change in the kernel module, see 282866. In the
kernel the issue has been more visible since the kernel might split the
messages in a quite arbitrary way. In userspace the issue has been
harder to notice, but with Marek's changes increasing the performance it
has become more urgent.

Change-Id: I993529a2f8e7f8e477733c929232b10fb981eaba