MINOR: ot: renamed the variable dbg_indent_level to flt_ot_dbg_indent_level

MINOR: ot: renamed the variable dbg_indent_level to flt_ot_dbg_indent_level

The thread-local variable dbg_indent_level used a generic name that could
collide with identifiers in other compilation units.  Renamed it to
flt_ot_dbg_indent_level so that it carried the flt_ot_ prefix consistent
with the rest of the OpenTracing filter namespace.  The rename covered the
declaration, definition, and all macro references in debug.h, parser.c and
util.c.