Do a blocking flush every 100 calls to put_copy_data

We had a blocking flush in pg-1.3.x at every call to put_copy_data.
This made sure, that all data is sent until the next put_copy_data.
In ged#462 (and pg-1.4.0 to .2) the behaviour was changed to rely on the non-blocking flushs libpq is doing internally.
This makes a decent performance improvement especially on Windows.
Unfortunately ged#473 proved that memory bloat can happen, when sending the data is slower than calls to put_copy_data happen.

As a trade-off this proposes to do a blocking flush only every 100 calls.

If libpq is running in blocking mode (PG::Connection.async_api = false) put_copy_data does a blocking flush every time new memory is allocated.
Unfortunately we don't have this kind of information, since we don't have access to libpq's PGconn struct and the return codes don't give us an indication when this happens.
So doing a flush at every fixed number of calls is a very simple heuristic.

Fixes ged#473

ext/pg_connection.c

@@ -266,6 +266,7 @@ pgconn_s_allocate( VALUE klass )
 	this->encoder_for_put_copy_data = Qnil;
 	this->decoder_for_get_copy_data = Qnil;
 	this->trace_stream = Qnil;
+	rb_ivar_set(self, rb_intern("@calls_to_put_copy_data"), INT2FIX(0));
 
 	return self;
 }

lib/pg/connection.rb

@@ -408,7 +408,17 @@ def isnonblocking
 	# See also #copy_data.
 	#
 	def put_copy_data(buffer, encoder=nil)
+		# sync_put_copy_data does a non-blocking attept to flush data.
 		until res=sync_put_copy_data(buffer, encoder)
+			# It didn't flush immediately and allocation of more buffering memory failed.
+			# Wait for all data sent by doing a blocking flush.
+			res = flush
+		end
+
+		# And do a blocking flush every 100 calls.
+		# This is to avoid memory bloat, when sending the data is slower than calls to put_copy_data happen.
+		if (@calls_to_put_copy_data += 1) > 100
+			@calls_to_put_copy_data = 0
 			res = flush
 		end
 		res
@@ -431,6 +441,7 @@ def put_copy_end(*args)
 		until sync_put_copy_end(*args)
 			flush
 		end
+		@calls_to_put_copy_data = 0
 		flush
 	end
 	alias async_put_copy_end put_copy_end