Logging: pass dynamic log level to common functions and subsystems

Logging: pass dynamic log level to common functions and subsystems

This patch makes the project's common functions and subsystems get a
dynamic log level instead of using a global one set from an environment
variable. The goal is to achieve per-component log levels, so the common
functions and subsystems must receive, eventually, the component's
specific log level.

"Naked" functions (without a context object of some sort) simply get
their log level from a new parameter. In that case `BT_LOG_OUTPUT_LEVEL`
is simply defined to `log_level`, so a local `log_level` variable must
exist when using logging statements. Subsystems, such as an FD cache or
a CTF serializer, get their log level from a new parameter in their
initialization function and then use it for their other operations.

It was somehow hard to do this for `src/common` alone without also doing
it for the rest so I'm doing all of them at once here.

For those subsystems, the local `logging.c` and `logging.h` disappear.

`tests/plugins/flt.lttng-utils.debug-info/test_bin_info.c` needs to pass
a log level to the FD cache subsystem; I used the WARN level instead of
setting the log level from an environment variable for this test alone.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I35979a8fe4c74afe5921c3b6651015cd7ee2760a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1460
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>