BRIN indexes cause server crash after continuous autovacuum

[robe]

Under intense conditions of autovacuuming of a geometry BRIN indexed tables, crashes have been found to occur.

Fixes suggested by Giuseppe Broccolo suggested

How to fix:

1. Define a MERGE support function and add it in the OpClass
2. Define a new UNION support function, similar to the ADD_VALUE one, that override the Postgresql core one in the OpClass (note that the MERGE support function is only used by ADD_VALUE and UNION)

Solution 2. is maybe the most consistent with the current implementation in PostGIS due to the peculiarity of geospatial data type in order to be indexed, that originally brought us to redefine (and simplify compared to Postgresql core version) the ADD_VALUE function. But I would like to hear about other opinions regarding this.