Implement LTTng-UST perf counters support

Introduce perf:cpu:* and perf:thread:* performance monitoring counter
contexts. perf:cpu: is used for kernel tracing, and perf:thread: is used
for UST. perf:* is kept for backward compatibility, but not shown in
lttng add-context --help anymore.

Example usages of performance counters have been adapted to this new
interface.

For the lttng.h API We introduce LTTNG_EVENT_CONTEXT_PERF_CPU_COUNTER
and LTTNG_EVENT_CONTEXT_PERF_THREAD_COUNTER as the API counterparts of
the new command line options. LTTNG_EVENT_CONTEXT_PERF_COUNTER is kept
for backward compatibility.

For ABIs with the tracers, introduce the new
LTTNG_UST_CONTEXT_PERF_THREAD_COUNTER to support this new UST feature.
Rename LTTNG_KERNEL_CONTEXT_PERF_COUNTER to
LTTNG_KERNEL_CONTEXT_PERF_CPU_COUNTER, following the same change in
lttng-modules (keeping the same enum entry value).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>