To represent a stream ID, Openfire defines the StreamID interface. However,
that interface is hardly used. Instead, simple String representations are used
in most places.

StreamIDs should conform to some specifications (notably: they shouldn't
include XML special characters). An implementation of the StreamID interface
is a good place to enforce specifications like that.

This commit replaces all String-based stream ID representations with an
implemenation of StreamID. For clustering/hazelcast, the serialized string
is still being used to exchange data between nodes (I expect that this allows
older and newer cluster nodes to continue to exchange cache information within
the same cluster).

Also in this commit:
- StreamID implementations check for (and escape) XML special characters.
- Duplicate implemenations of the StreamID interface are removed.

Some code refers to streamID while in reality session IDs are implemented. This
commit does not affect those pieces of code.