This commit moves the location of the invoice counter key which is an
auto-incrementing primary key for all invoices. Rather than storing the counter
in the same top-level invoice bucket, the counter is now stored within the
invoiceIndex bucket. With this change, the top-level bucket can now cleanly be
scanned in a sequential manner to retrieve all invoices.
This commit adds the necessary database functionality required for a
high-level payment invoice workflow. Invoices can be added dealing the
requirements for fulfillment, looked by payment hash, and the finally
also settled by payment hash. For record keeping and the possibility of
reconciling future disputes, invoices are currently never deleted from
disk. Instead when an invoice is settled a bit is toggled indicating as
much.
The current invoiceManger within the daemon will be modified to use
this persistent invoice store, only storing certain “debug” invoices in
memory as dictated by a command line flag.
