Refactor some JetStream helper code, add support for specifying JetStream domain #123

Job	Run time
Linting	1m 50s
Build for Linux (linux, amd64)	55s
Unit tests	4m 5s
Build for Linux (linux, 386)	1m 9s
Build for Windows (windows, amd64)	1m 59s
WASM build test	0s
Initial tests passed	2s
Upgrade tests from HEAD-2	7m 5s
Integration tests	5m 25s
Sytest (SQLite native)	5m 1s
Complement (SQLite native)	4m 10s
Upgrade tests	7m 0s
Sytest (SQLite Cgo)	5m 24s
Complement (SQLite Cgo)	4m 35s
Sytest (PostgreSQL)	6m 50s
Complement (PostgreSQL)	9m 37s
Integration tests passed	2s
Update Docker images	0s
	1h 5m 9s

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