Replace usage of URI class to identify endpoint.

[sbernard31]

Since we introduce the new transport layer abstraction, we identify endpoint by URI using java.net.URI.

There is some argument to replace this by a String or a dedicated new class (EndpointURI ?).

Here is a list of those arguments :

• java.net.URI use lot of memory : Should Registration be strictly tied to a server endpoint ? #1415 (comment)
• java.net.URI is full of not fixed bug and doesn't support RFC3986 and RFC3987 : https://cr.openjdk.org/~dfuchs/writeups/updating-uri/
• java.net.URI is overkill, currently we only need scheme, host and port.

Spring project goes to the conclusion that URI class has lot of issue and should be replaced :

In fact, one of the reasons for introducing UriComponents in Spring Framework was because java.net.URI had known encoding issues, which meant that we had to create our own abstraction that did follow the related RFCs. And as mentioned above, accepting the % is one thing, but it really should be encoded on output, in accordance with RFC 6874.

(The % point issue is relative to Leshan issue : #1580 (comment))

[sbernard31]

It should be fixed by #1648 (will be available in 2.0.0-M17)