Remove unused imports

Some tests contained unused imports. This commit removes these imports.
Convert fields to local variables

Some test classes contain fields which were only used in one spot. This
commit converts these fields to local variables.
Remove redundant local variable

Remove redundant local variable from test utility class.
Add explicit version to maven-gpg-plugin

The maven-gpg-plugin declaration didn't feature a version number. This
could cause trouble if an update is published which has breaking
changes. This commit adds an explicit version number so the external
dependency can no longer break our build.
Add DataTransferMessage type

In an attempt to lower code duplication and unit interfacing this commit
introduces the DataTransferMessage type which bundles the vendor
identifier, message identifier, and data for data transfer related
commands and events. This commit only introduces the type and uses it in
the IncomingDataTransferCommand and all the code interacting with this
command.