Refactor some JetStream helper code, add support for specifying JetStream domain #121

Job	Run time
Linting	1m 12s
Build for Windows (windows, amd64)	1m 22s
Unit tests	1m 23s
WASM build test	0s
Build for Linux (linux, amd64)	1m 4s
Build for Linux (linux, 386)	1m 12s
Initial tests passed	1s
Integration tests	1s
Upgrade tests	1s
Complement (${{ matrix.label }})	1s
Sytest (${{ matrix.label }})	1s
Upgrade tests from HEAD-2	1s
Integration tests passed	1s
Update Docker images	1s
	6m 21s

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