Cleanup parallel BRIN index build code

Cleanup parallel BRIN index build code

Commit b43757171470 added support for parallel builds of BRIN indexes,
using code similar to BTREE. But there were to be a couple unnecessary
differences, particularly in how the leader waits for the workers, and
merges the results. So remove these, to make the code more similar.

The leader never waited on the workersdonecv condition variable, but
simply called WaitForParallelWorkersToFinish() in _brin_end_parallel()
and then merged the per-worker results. This worked correctly, but it
seems better to do the wait and merge before _brin_end_parallel().

This commit moves the relevant code to _brin_parallel_heapscan/merge(),
which means  _brin_end_parallel() remains responsible only for exiting
the parallel mode and accumulating WAL usage data.

Discussion: https://postgr.es/m/3733d042-71e1-6ae6-5fac-00c12db62db6@enterprisedb.com