Abstract "base" condition code in a separate class

Each condition has an underlying lock, the acquire and release methods,
and a few helper methods to check that it's called in the proper way.

Abstract them to a separate class so we can have more than one without
duplicating this code.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>