extsearch: support publicinbox.*.boost parameter

This behaves identically the lei external "boost" parameter in
prioritizing raw messages for extindex.

Relying exclusively on the config file order doesn't work well
for mirrors since it's impossible to guarantee config file
ordering via grokmirror hooks.

Config file ordering remains the default if boost is
unconfigured, or in case of ties.

Note: I chose the name "boost" rather than "priority" or "rank"
since I always get confused by whether higher or lower numbers
take precedence when it comes to kernel scheduling.  "weight" is
also a part of Xapian API terminology, which we currently do not
expose to configuration (but may in the future).

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