Previously various parts of the code routed size requests through
RelationGetNumberOfBlocks[InFork]. That works if md.c is used by the
AM, but not otherwise.

Add a tableam callback to return the size of the table. As not every
AM will use postgres' BLCKSZ, have it return bytes, and have
RelationGetNumberOfBlocksInFork() round the byte size up into blocks.

To allow code outside of the AM to determine the actual relation size
map InvalidForkNumber the total size of a relation, as not every AM
might just need the postgres defined forks.

A few users of RelationGetNumberOfBlocks() ought to be converted away
from that. One case, the use of it to determine whether a tid is
valid, will be fixed in a follow up commit. Others will have to wait
for v13.

Author: Andres Freund
Discussion: https://postgr.es/m/20190423225201.3bbv6tbqzkb5w7cw@alap3.anarazel.de