pgsql: Move some md.c-specific logic from smgr.c to md.c.

Move some md.c-specific logic from smgr.c to md.c.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dfd0121dc73aab491bcaad2d2b7a2a749389add8

Modified Files
--------------
src/backend/storage/smgr/md.c   | 37 +++++++++++++++++++++++++++++++------
src/backend/storage/smgr/smgr.c | 33 ++++-----------------------------
src/include/storage/md.h        |  1 +
3 files changed, 36 insertions(+), 35 deletions(-)