Potential future SMGR implementations may not want to create
tablespace directories when creating an SMGR relation.  Move that
logic to mdcreate().  Move the initialization of md-specific
data structures from smgropen() to a new callback mdopen().

Author: Thomas Munro
Reviewed-by: Shawn Debnath (as part of an earlier patch set)
Discussion: https://postgr.es/m/CA%2BhUKG%2BOZqOiOuDm5tC5DyQZtJ3FH4%2BFSVMqtdC4P1atpJ%2Bqhg%40mail.gmail.com