global: make use of FunctionalInterface

[grimreaper]

Now that we're using Java 8, it seems reasonable to annotate.

[soc]

@grimreaper Would it make sense in general to also annotate interfaces that just duplicate the ones in java.util.function with @Deprecated?

[grimreaper]

IMHO yes, though that should be done separately.

[kinow]

Good idea. I think it doesn't break backward compatibility, and would indicate that it is indeed a functional interface, make sure it keeps being one (i.e. no more methods added without an error).

Let's see what others think, but +1 from me 👍

[coveralls]

Coverage remained the same at 87.824% when pulling 598a42b on grimreaper:eax/finter/now into ca6851e on apache:master.