This feature was using a process local map to track the first few blocks
in the relation.  The map was reset each time we get the block with enough
freespace.  It was discussed that it would be better to track this map on
a per-relation basis in relcache and then invalidate the same whenever
vacuum frees up some space in the page or when FSM is created.  The new
design would be better both in terms of API design and performance.

List of commits reverted, in reverse chronological order:

06c8a509  Improve code comments in b0eaa4c5.
13e8643b  During pg_upgrade, conditionally skip transfer of FSMs.
6f918159  Add more tests for FSM.
9c32e4c3  Clear the local map when not used.
29d108cd  Update the documentation for FSM behavior..
08ecdfe7  Make FSM test portable.
b0eaa4c5  Avoid creation of the free space map for small heap relations.

Discussion: https://postgr.es/m/20190416180452.3pm6uegx54iitbt5@alap3.anarazel.de