It's a common pattern to look for all services hosted under the users
service that provide a given feature. Let's provide an API for that and
use that API in the methods that benefit from it.
and MultipleRecipient (XEP-33) related code in general (e.g. use
constants where possible).
For getMultipleRecipientServiceAddress() this means that
- the "synchronized (services)" block is removed
- ServiceDiscoveryManager is only retrieved once
instead of using the old baseName=smack appendix=project.name approach,
we are now going convention over configuration and renaming the
subprojects directories to the proper name.
Having a prefix is actually very helpful, because the resulting
libraries will be named like the subproject. And a core-4.0.0-rc1.jar is
not as explicit about what it actually *is* as a
smack-core-4.0.0-rc1.jar.
SMACK-265
