Introduce a "log level" support for DEBUG_TYPE (#150855)

This allows to set an optional integer level for a given debug type. The
string format is `type[:level]`, and the integer is interpreted as such:

- if not provided: all debugging for this debug type is enabled.
- if >0: all debug that is < to the level is enabled.
- if 0: same as for >0 but also does not disable the other debug-types,
it acts as a negative filter.

The LDBG() macro is updated to accept an optional log level to
illustrate the feature. Here is the expected behavior:

LDBG() << "A"; // Identical to LDBG(1) << "A";
LDBG(2) << "B";

With `--debug-only=some_type`: we'll see A and B in the output.  
With `--debug-only=some_type:1`: we'll see A but not B in the output. 
With `--debug-only=some_type:2`: we'll see A and B in the output. (same
with any level above 2)
With `--debug-only=some_type:0`: we'll see neither A nor B in the
output, but we'll see any other logging for other debug types.