Move CustomerFwd behind Customer

All the previously-lazy BackendSgx data is now either all loaded or all not
loaded by swapping the sgx_repo used by your CustomerRepo instance.  When not
loaded the fields are filled with bottom values that explode when used.  When
loaded the values are present in RAM and not promises at all.  Most code paths
do not need any of the data, a few need most of it, so this seems like a good
trade-off.  Most code using this object will simply never touch those fields or
care about how they are loaded, etc.

Of course, most of this data isn't even SGX related and should move out of here,
but that would take a data model refactor/migration on the catapult_* schema.

Merge branch 'create_customer_id'

* create_customer_id:
  Create customer_id if it does not exist before we start registration
  Break out CustomerPlan
  Inject BackendSgx per customer

Inject BackendSgx per customer

Instead of being a singleton that represents the entire relationship
with the backend, the object is now per-customer (since any meaningful
method requires a customer anyway for the from JID at least) and can be
delegated to directly from Customer.

Merge branch 'new-signup-add-credit-card'

* new-signup-add-credit-card:
  Happy path for credit card signup
  Panic should work on any value for error
  Allow getting default payment method, not just index
  Object representing the backend SGX to use
  Stop polluting Object namespace with Blather DSL
  Work in the presence of em-synchrony
  Helper to allow ordering phone number from Bandwidth v2
  Helper to get a promise that resolves after N seconds
  Every payment kind will need the plan, so put it at the top
  Use registration pattern for Payment kinds
  Method to bill the plan of a Customer
  Use Forwardable for simple delegations
  Helper to allow using sync-style code in a Promise context
  New signup: go to web to choose credit card
  OOB helper
  Reject promise on stanza error
  Fix typo

Object representing the backend SGX to use

This is what should know about things like the customer id -> jid mapping, etc.