Move the dirty queues inside the per-domain structure. This resolves a bug
where we had not hit global dirty limits but a single queue was starved
for space by dirty buffers. A single buf_daemon is maintained for now.

Add a bd_speedup() when we are low on bufspace. This can happen due to SUJ
keeping many bufs locked until a cg block is written. Document this with
a comment.

Fix sysctls to work with per-domain variables. Add more ddb debugging.

Reported by: pho
Reviewed by: kib
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14705