uhd: fixes for time_spec's to_ticks and from_ticks calls

The fixes address rouding issues when the tick_rate is a non-integer.

Conflicts:

	host/tests/time_spec_test.cpp
uhd: switch the reusable barrier to condition variables

This allows the converter threads in a multi-threaded streamer to wait quietly.
In addition, the use of two barriers in the packet handlers was reduced to one,
by adding a simple exit barrier inside the reusable barrier's wait method.