Add Connection#send_pipeline_sync, async_pipeline_sync and release GVL at PQ(sendP|p)ipelineSync #752

Job	Run time
Build	4s
Test (windows, head, 17.0-1-windows-x64, 17)	4m 5s
Test (windows, 2.7, 10.20-1-windows-x64, 10)	4m 3s
Test (windows, mswin, 17.0-1-windows-x64, 17)	4m 9s
Test (ubuntu, head, 17)	1m 8s
Test (ubuntu, 3.2, 12)	1m 17s
Test (ubuntu, 20.04, 2.7, 10)	1m 31s
Test (ubuntu, truffleruby, 13)	43s
Test (ubuntu, truffleruby-head, 17)	1m 47s
Test (macos, head, 17.0-1-osx, 17)	1m 22s
	20m 9s

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