Move gel-connpool from edgedb repo #614

Job	Run time
Generate matrix	7s
linux / check / rust=beta	5s
macos / check / rust=stable	13s
macos / test / rust=stable	1m 39s
windows / check / rust=stable	12m 16s
linux / check-fast / rust=stable	5s
linux / check / rust=stable	10s
linux / test-fast / rust=beta	10s
linux / check / rust=default	10s
linux / test / rust=stable	10s
linux / build-minimal / rust=default	6s
windows / test-fast / rust=stable	20m 4s
linux / test-fast / rust=default	7s
	35m 22s

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