Propagate parallel VACUUM's buffer access strategy.

Propagate parallel VACUUM's buffer access strategy.

Parallel VACUUM relied on global variable state from the leader process
being propagated to workers on fork().  Commit b4af70cb removed most
uses of global variables inside vacuumlazy.c, but did not account for
the buffer access strategy state.

To fix, propagate the state through shared memory instead.

Per buildfarm failures on elver, curculio, and morepork.

Many thanks to Thomas Munro for off-list assistance with this issue.