Consensus method 18: Add a base64 ID digest to the microdesc

This is a stopgap measure to make sure that microdescriptors never
collide; see bug 11743.
Use siphash on channel/circuit-id map too

Fixes ticket 11750.
Faster chan_circid_entry_hash implementation

Since this is critical-path, let's tune the value we pass to
csiphash a little so it fits into one whole round.
Add a siphash benchmark.
changes file for bug11743
fix a copy-paste comment mistake
Future-proof "id" lines against proposal 220.
Unit test for dirvote_create_microdescriptor
Merge branch 'bug11743_option_b'
Merge remote-tracking branch 'public/bug11750'
Remove a spurious variable.