The events listener now uses a proper self-consuming queue and
the event handlers have been rewritten accordingly.
The way this works is that running init() on the handlers
library instantiates the handlers and returns two higher-order
functions, prepare() and despatch(). A call to the latter of
these is appended to the queue with each new incoming event.
The handlers have access to a context object (ctx) which may be
used to persist data between calls and/or exchange data between
handlers. This is used notably to give the handlers access to
project configurations, which are themselves refreshed by a
project configuration change handler (DetectProjectConfigurationChange).
