Changes

After writing VTY config, run sync() in a different thread

Running sync() after writing VTY config causes the process to stop for
a fraction of a second. This may result in delayed processing that
causes buffer underruns, overflows and delay. This is a problem with
applications that do real-time signal processing such as osmo-trx and
osmo-bts.

After some discussions, I decided to simply remove the sync() commands
from the VTY's write function. There is still a backup file created
of the previously saved configuration before writing the current
configurtion. Loosing current configuration caused by a system crash
afterwards is very unlikely with modern file systems.

Related: OS#6438
Change-Id: I3cb2ee68b2e4c730f96522208c4abf00d0f49a44