Keep message in order even when save-fork-late-message-in-db=true

ForkContext do not call directly Agent::injectRequestEvent but ModuleRouter::injectRequestEvent. Then ModuleRouter uses Injectors to send the request. Depending on configuration it can use AgentInjector (no saves in DB, message order is already preserved) or ScheduleInjector (when messages are saved in DB, in order to preserve order).

(cherry picked from commit 41bf8240)

See !760 (merged)