This is a follow-up of the work done in e3931d01.  This case is a bit
different than pg_attribute and pg_shdepend: the maximum number of items
to insert is known in advance, but there is no need to handle pinned
dependencies.  Hence, the base allocation for slots is done based on the
number of items and the maximum allowed with a cap at 64kB.  Slots are
initialized once used to minimize the overhead of the operation.

The insertions can be done for dependencies of the same type.  More
could be done by grouping the insertion of multiple dependency types in
a single batch.  This is left as future work.

Some of the multi-insert logic is also simplified for pg_shdepend, as
per the feedback discussed for this specific patch.  This also moves to
indexing.h the variable capping the maximum amount of data that can be
used at once for a multi-insert, instead of having separate definitions
for pg_attribute, pg_depend and pg_shdepend.

Author: Daniel Gustafsson, Michael Paquier
Reviewed-by: Andres Freund, Álvaro Herrera
Discussion: https://postgr.es/m/20200807061619.GA23955@paquier.xyz