Rename consumer threads and spawn them in daemon

The metadata thread is now created in the lttng-consumerd daemon so all
thread could be controlled inside the daemon.

This is the first step of a consumer thread refactoring which aims at
moving data and metadata stream operations inside a dedicated thread so
the session daemon thread does not block and is more efficient at adding
streams.

The most important concept is that a stream file descriptor MUST be
opened as quickly as we can then passed to the right thread (for UST
since they are already opened by the session daemon for the kernel).

Signed-off-by: David Goulet <dgoulet@efficios.com>