fix 'cache lookup failed for function 0' errors during initdb

fix 'cache lookup failed for function 0' errors during initdb

Commit 0e882bd2e02 broke planning for aggregates not supporting partial
aggregation (e.g. array_agg, which lacks the serialize, deserialize and
combine functions). The code attempted to initialize the costing info
anyway, which failed when attempting to fetch info about those functions
from syscache.

Introduce a new flag 'try_distributed_aggregation' akin to the existing
one for simple parallel case. This is a good idea anyway, because it'll
allow us to disable distributed paths if we happen to find out the whole
aggregate can be pushed down.