Allows creation a Multi with an emitter and a buffer size

[cescoffier]

At the moment, you can create a Multi from an emitter but you can only configure the back pressure strategy. When using "buffer", it uses the default buffer size. We should allow configuring the buffer size.

Now, the question is what happens if the buffer reaches its full capacity:

1. should the sender be informed (throwing an exception) - the event is dropped
2. should the downstream be informed, and so no more event would be received
3. all the above

[cescoffier]

Going with (1) as with it, the producer can decide what needs to be done (send a failure, completion, wait until there is more requests).

[cescoffier]

Actually, the async and serialized nature of the emitter does not allow throwing an exception, as it may be thrown from another thread. So, going with 2).

Fortunately, for 1) the caller can check if the amount of request is greater than 0.