Adds `Last-Modified` header support and enables this (and `ETag`s)
by default for rtrmon.
I'm using rtrmon with a backend that suports `If-Modified-Since`
but not `If-None-Match`. To poll this often (sub-minute interval)
it is better to use conditional requests.
Validators (such as rpki-client) ingest ROAs and emit Validated ROA Payloads (VRPs).
RTR servers exclusively deal with ingesting VRPs and emitting VRPs via RTR.
