Fix dynahash's HASH_FIXED_SIZE ("isfixed") option.

Fix dynahash's HASH_FIXED_SIZE ("isfixed") option.

This flag was effectively a no-op in EXEC_BACKEND (ie, Windows)
builds, because it was kept in the process-local HTAB struct,
and it could only ever become set in the postmaster's copy.

The simplest fix is to move it to the shared HASHHDR struct.
We could keep a copy in HTAB as well, as we do with keysize
and some other fields, but the "too much contention" argument
doesn't seem to apply here: we only examine isfixed during
element_alloc(), which had better not get hit very often for
a shared hashtable.

This oversight dates to 7c797e719 which invented the option.
But back-patching doesn't seem appropriate given the lack of
field complaints.  If there is anyone running an affected
workload on Windows, they might be unhappy about the behavior
changing in a minor release.

Author: Aidar Imamov <a.imamov@postgrespro.ru>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/4d0cb35ff01c5c74d2b9a582ecb73823@postgrespro.ru