Lessen stateful Zinc overhead

[pauldraper]

Stateful incremental Zinc is disabled by default. However, there still extra work being done just because it is an option. E.g. unpacking upstream JARs.

There should be a choice between two modes (probably toolchains).

(a) Stateless. Does not unpack JARs, etc.

(b) Stateful-optional. Does unpack JARs, etc. Can use prior state if the --persistent_dir flag is specified.

These two modes would produce different outputs and so would not share cache hits between them. (Though within the mode, they would share cache hits.)