Rename VERBOSE log level to TRACE

Some people reported that they have a hard time guessing whether the
VERBOSE log level is more or less verbose than the DEBUG log level.

The existing logging frameworks I looked at (Apache Log4j 2, SLF4J,
`java.util.logging`, syslog, Python, Ruby, Boost.Test, ASP.NET Core, to
name a few popular ones) do not have the VERBOSE level; they almost
always have the DEBUG level, and sometimes the TRACE level.

To conform with the common practice, let's use TRACE instead of VERBOSE.
It is my wish that DEBUG vs. TRACE will speak to more developers than
DEBUG vs. VERBOSE.

This means that BT_LOGV() and its variants become BT_LOGT(). Also,
the log level environment variables and the CLI's `--log-level` parameter
accept `T` and `TRACE` instead of `V` and `VERBOSE`. In the log output,
the messages contain `T` instead of `V`:

    06-20 14:31:23.356  3612  3612 T PLUGIN/CTF/BFCR
    read_unsigned_bitfield@bfcr.c:483 [N/A] Read unsigned bit array:
    cur=32, size=8, bo=1, val=33

`CONTRIBUTING.adoc` is updated to document the TRACE level instead of
the VERBOSE level.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I20460c4569f8b755440511c6621a68a1ef6424fc
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1524
CI-Build: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Jonathan Rajotte Julien <jonathan.rajotte-julien@efficios.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>