lttng_ust_init_thread: initialise cached context values

Modify all relevant *_alloc_tls functions so that they take
flags for 'init'. Rename them to init_thread for consistency.

So far define one flag LTTNG_UST_INIT_THREAD_CONTEXT_CACHE,
this will warm up cached values so less is done during
the first tracepoint.

The function 'lttng_ust_init_thread' will use all available
flags, software can opt-in to do work early instead
of lazily during tracepoints.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>