Remove pg_vlock locking from VACUUM, allowing multiple VACUUMs to run in

Remove pg_vlock locking from VACUUM, allowing multiple VACUUMs to run in
parallel --- and, not incidentally, removing a common reason for needing
manual cleanup by the DB admin after a crash.  Remove initial global
delete of pg_statistics rows in VACUUM ANALYZE; this was not only bad
for performance of other backends that had to run without stats for a
while, but it was fundamentally broken because it was done outside any
transaction.  Surprising we didn't see more consequences of that.
Detect attempt to run VACUUM inside a transaction block.  Check for
query cancel request before starting vacuum of each table.  Clean up
vacuum's private portal storage if vacuum is aborted.