Partial implicit/explicit withdrawals

[vkotronis]

Is your feature request related to a problem? Please describe.
Withdrawals related to subsets of hijacked IP address space should lead to partial withdrawal characterization for the monitors that saw them. For this we also need new persistent state.

Describe the solution you'd like
For each monitor/peer that saw an implicit/explicit BGP withdrawal for a hijacked prefix, we should keep state as follows:

Peer X - Subprefix P withdrawn - Progress Y%

E.g., if prefix 10.0.0.0/8 is hijacked and via more specifics the subprefix 10.0.0.0/9 is correctly routed as seen on an initially infected monitor, then the hijack is 50% withdrawn on the side of the monitor.

Describe alternatives you've considered
N/A

Additional context
See also #438