We used to use a bitfield there, which was restricting the total
number of flags to 32 (unless we switch to uint64, which is suboptimal).
Moreover, with the introduction of the global debug flags, we have
even fewer bits to flip, and this shared nature was restricting
the introduction of more global debug channels.
The goal for the bitfield was to make it possible to put a debug message into
more than one channel. But this feature was practically not used by the engines.
This was removed by the previous commits.
This commit turns the debug channels into a hashmap and puts the global channel
IDs after 100000.
There is no absolute need to renumber the existing debug channels, but
I could follow with an engine or two.
bf3117e935