clock override: introduce getter API for lttng tools

clock override: introduce getter API for lttng tools

Currently, the lttng session daemon reimplements its own copy of
struct lttng_trace_clock, which is pretty bad as ABI go.

Implement getter functions for each of the clock callback instead. This
refactoring needs to be introduced with a matching commit to use the new
ABI in LTTng tools.

Move the implementation of the clock frequency, uuid, name and
description default callbacks (for monotonic clock) to lttng-ust.

This allow to hide the previously exposed "lttng_trace_clock" public
symbol. Rename this internal symbol to lttng_ust_trace_clock for prefix
consistency. Rename struct lttng_trace_clock to struct
lttng_ust_trace_clock for similar reasons.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I55825f0c0f5372a68aaaf06fb8dbd256cf248f2c