For now, we only have one concrete implementation in sourcekit-lsp
itself, but it is useful for libraries to be able to wrap their own
sourcekitd implementation for example if they have an existing external
use of sourcekitd in C code.
The notification handler and initialize/shutdown APIs all modify global
state within the service, so handle them inside the wrapper to prevent
accidental misuse. Also add support for multiplexing the notification
handler, since it can be useful to have multiple handlers, especially
for global notifications.
