Adding read-only & disabled mode for Logplex API

The Disabled mode entirely shuts down access to the web API at all
levels, including for healthchecks. Use with extreme care.

The read-only mode forbids all non-GET operations outside of what v2
canaries are allowed to do.

Read-Only mode should be something usable for maintenance operations where the central
config database needs to be changed, taken down, swapped, etc. without running into
unexpected errors or inconsistent data.