Refactor code for reading and writing relation map files.

Refactor code for reading and writing relation map files.

Restructure things so that the functions which update the global
variables shared_map and local_map are separate from the functions
which just read and write relation map files without touching any
global variables.

In the new structure of things, write_relmap_file() writes a relmap
file but no longer performs global variable updates. A symmetric
function read_relmap_file() that just reads a file without changing
any global variables is added, and load_relmap_file(), which does
change the global variables, uses it as a subroutine.

Because write_relmap_file() no longer updates shared_map and
local_map, that logic is moved to perform_relmap_update(). However,
no similar logic is added to relmap_redo() even though it also calls
write_relmap_file(). That's because recovery must not rely on the
contents of the relation map, and therefore there is no need to
initialize it. In fact, doing so seems like a mistake, because we
might then manage to rely on the in-memory map where we shouldn't.

Patch by me, based on earlier work by Dilip Kumar. Reviewed by
Ashutosh Sharma.

Discussion: http://postgr.es/m/CA+TgmobQLgrt4AXsc0ru7aFFkzv=9fS-Q_yO69=k9WY67RCctg@mail.gmail.com