Remove duplicate block()/unblock() code

This makes all EMPER synchronization primitives inherit from
Blockable. As result all block and unblock operations of a context
will go to the same code path. We can later use this to store the set
of currently blocked contexts.

Also Blockable now inherits Logger, which makes some things easier,
but as result we have to invoke the logX() methods with "this->"
prepended. See related code comment.