This commit enables parallel writes for Foreign Data Wrapper. This
feature is currently missing from the FDW framework, whilst parallel
scans are supported, parallel writes are missing. FDW parallel writes
are analogous to writing to writable external tables that run on all
segments.

One caveat is that in the external table framework, writable tables
support a distribution policy:

    CREATE WRITABLE EXTERNAL TABLE foo (id int)
    LOCATION ('....')
    FORMAT 'CSV'
    DISTRIBUTED BY (id);

In foreign tables, the distribution policy cannot be defined during the
table definition, so we assume random distribution for all foreign
tables.

Parallel writes are enabled when the foreign table's exec_location is
set to FTEXECLOCATION_ALL_SEGMENTS only. For foreign tables that run on
master or any segment, the current policy behavior remains.