ISPN-2281 Convert REST server to store data passing in MIME information

* Created REST MimeMetadata class implementing Metadata to pass in
content type information.
* Added putAsync(K, V, Metadata).
* Remove lifespanTimeUnit and maxIdleTimeUnit from Metadata interface
which are not really used. Metadata implementers should provide
lifespan/maxIdle times in milliseconds.
* Fixed bug where Metadata*Entry instances were not correctly
determined when using a custom Metadata implementation. Added tests
for it too.
* Make all Metadata*Entry instances return the metadata in their cache
value, and not null.
* Remove unused key equivalence check in Hot Rod server.