ctf: remove strict metadata mode, update automatic CC mapping behaviour

ctf: remove strict metadata mode, update automatic CC mapping behaviour

Update the automatic mapping of specific fields to clock classes when
no explicit mapping is found in the integer field type to follow this:

                     | 0 CCs             | 1 CC             | 2+ CCs
    --------------------------------------------------------------------
    FT is mapped     | Unreachable       | OK as is         | OK as is
    FT is not mapped | Map to auto 1 GHz | Map to single CC | Error

The (FT is not mapped, 0 CC) cell follows Babeltrace 1's behaviour: when
a special timestamp field is found in the packet context
(`timestamp_begin` or `timestamp_end`) or event header (`timestamp`)
scope, when this timestamp field is not explicitly mapped to a clock
class, and when there's no clock class at this point in the trace, then
create an automatic, 1 GHz clock class and, in this case, name it
`default`. Map the field to this implicit 1 GHz clock class. See
section 8 ("Clocks") of CTF v1.8.2.

Cell (FT is not mapped, 0 CC) is a fix for old versions of LTTng, and
cell (FT is not mapped, 1 CC) is a fix for old versions of CTF writer.
Cell (FT is not mapped, 2+ CCs) is an error because we wouldn't know
which clock class to automatically map the field to.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>