Freeze enforcing: (RIPD-399)

 * Set enforce date: September 15, 2014
 * Enforce in stand alone mode
 * Enforce at source
 * Enforce intermediary nodes
 * Enforce global freeze in get paths out
 * Enforce global freeze in create offer
 * Don't consider frozen links a path out
 * Handle in getBookPage
 * Enforce in new offer transactors
How big a pain is it to use an enum instead of a bool here?  The methods all have named parameters, but here at the call site it's hard to know what that appended bool is intended to do.  Yeah, that suggestion would reverberate through the code.  It may be too much work.