Implement lock for GridcoinConnectBlock to queued txindex write. #1793

Job	Run time
Lint	43s
ARM [GOAL install] [buster]	7m 9s
Win32	8m 15s
macOS 12 native [GOAL install] [GUI] [no depends]	12m 6s
Win64	11m 11s
i386 Linux [GOAL install] [focal]	7m 5s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	15m 12s
x86_64 Linux [GOAL install] [GUI] [bionic] [no depends]	14m 30s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [focal]	9m 18s
	1h 25m 29s

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