feat(loadgen): implement load test harness for messaging workers #128

Job	Run time
prewarm	17s
test	3m 23s
lint	35s
test-integration (notification-worker)	1m 10s
test-integration (pkg/userstore)	54s
test-integration (inbox-worker)	1m 9s
test-integration (pkg/roomcrypto)	57s
test-integration (pkg/natsrouter)	1m 14s
test-integration (message-worker)	2m 16s
test-integration (pkg/roomkeysender)	1m 27s
test-integration (auth-service)	58s
test-integration (pkg/roomkeystore)	58s
test-integration (history-service)	2m 11s
test-integration (room-service)	1m 26s
test-integration (broadcast-worker)	1m 15s
test-integration (tools)	1m 19s
test-integration (search-sync-worker)	2m 29s
test-integration (room-worker)	1m 12s
	25m 10s

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