Push down joins to the remote MongoDB servers.

If we have a join between two foreign tables from the same remote
server, push that join down to the remote server instead of fetching
all the rows for both the tables and performing a join locally.  This
will have a very significant win in many practical cases.

Since it is very unclear that which of the operators are supported on
the MongoDB servers, currently join involving only relational and
arithmetic operators in join-clauses are pushed down to avoid any
potential join failure.  Also, only INNER and LEFT/RIGHT OUTER
joins are supported, and not FULL, SEMI, and ANTI joins.  Moreover,
only joins between two tables are pushed down and not when either
inner or outer relation is the join itself.

We use an aggregate pipeline with the lookup operator.  Also, the
MongoDB server returns join results as an array, and thus to get in
a tuple form we use the unwind operator.

This patch also supports the partition-wise join push-down when the
table is partitioned.

FDW-138, Vaibhav Dalvi, reviewed by Suraj Kharage and Jeevan Chalke,
with additional improvements by me. Tested by Rajkumar Raghuwanshi.

Makefile.meta

@@ -20,7 +20,7 @@ MONGO_INCLUDE = $(shell pkg-config --cflags libmongoc-1.0)
 PG_CPPFLAGS = --std=c99 $(MONGO_INCLUDE) -I$(LIBJSON) -DMETA_DRIVER
 SHLIB_LINK = $(shell pkg-config --libs libmongoc-1.0)
 
-OBJS = connection.o option.o mongo_wrapper_meta.o mongo_fdw.o mongo_query.o $(LIBJSON_OBJS)
+OBJS = connection.o option.o mongo_wrapper_meta.o mongo_fdw.o mongo_query.o deparse.o $(LIBJSON_OBJS)
 
 
 EXTENSION = mongo_fdw