Deprecate `javax.portlet:portlet-api` and portlet support

[cyb3r4nt]

JsonRpcServer depends on the javax.portlet:portlet-api and requires this to be in the classpath even if portlet support is not used.
At the same time, portlet applications are not very popular nowadays,
and probably there are no many users who actually need this.
My personal opinion is that majority of users do not use that feature,
and are obligated to bring javax.portlet:portlet-api only because jsonrpc4j needs this.

Please comment and vote if you still need that Java portlets support.

Portlet methods are actually quite short:
https://github.com/briandilley/jsonrpc4j/blob/1.6.0/src/main/java/com/googlecode/jsonrpc4j/JsonRpcServer.java#L76-L105

Active users may just inline this logic into their own projects,
and use other streaming method to handle those requests:
https://github.com/briandilley/jsonrpc4j/blob/1.6.0/src/main/java/com/googlecode/jsonrpc4j/JsonRpcBasicServer.java#L230-L260

The plan may be to mark it as @Deprecated(forRemoval = true),
and remove this in one of the future versions.

[vtintillier]

Should this be closed as fixed?