Set io_priority properly for all requests

Notably, changes wasn't calling set_io_priority so was not affected by
maintenance_mode.
Fix `fabric:get_db_info/1`

We weren't properly removing shards that had an error as well as
returning errors to functions that don't expect them.
Allow shard replacements in stream_start

This adds fabric_util:stream_start/4 which accepts a start function and
a proplist of replacement shards keyed by range. If a worker exits due
to maintenance mode and prevents progress from being made, the list of
replacement shards is searched for any replacements for the range. If
any are found the start function is called for each possible
replacement. The start function takes the possible replacement and
should return the new worker.

BugzId: 20423
Add utility function to find shard replacements

Streaming RPC coordinators can use this function to generate a list of
shard replacements that is suitable to be passed to
fabric_util:stream_start/4.

BugzId: 20423
Provide replacement shards in map views

BugzId: 20423
Provide replacement shards in reduce views

BugzId: 20423
Provide replacement shards in changes feeds

BugzId: 20423