Refactor autoprewarm_database_main() in preparation for read stream

Autoprewarm prewarms blocks from a dump file representing the contents
of shared buffers at the time it was dumped. It uses a sorted array of
BlockInfoRecords, each representing a block from one of the cluster's
databases and tables.

autoprewarm_database_main() prewarms all the blocks from a single
database. It is optimized to ensure we don't try to open the same
relation or fork over and over again if it has been dropped or is
invalid. The main loop handled this by carefully setting various local
variables to sentinel values when a run of blocks should be skipped.

This method won't work with the read stream API. The read stream
callback must be able to advance the current position in the
BlockInfoRecord array to allow for reading ahead additional blocks,
however a read stream maps 1-1 with a relation and fork combination. So,
the main loop in autoprewarm_database_main() must also advance the
position in the array of BlockInfoRecords to skip invalid relations and
forks. This split control doesn't fit well with the current flow control
in autoprewarm_database_main()

To make it compatible with the read stream API, change
autoprewarm_database_main() to explicitly fast-forward in the
BlockInfoRecords array past the blocks belonging to an invalid relation
or fork.

This commit only implements the new control flow -- it does not use the
read stream API.

Co-authored-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Co-authored-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/flat/CAN55FZ3n8Gd%2BhajbL%3D5UkGzu_aHGRqnn%2BxktXq2fuds%3D1AOR6Q%40mail.gmail.com

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