Closes: Codaone#442

Purge saved virtual orders only if no buy AND sell orders. If we have
both buy and sell real orders, restore both. If we have only one type of
orders, restore corresponding virtual orders and purge opposite orders.

AttributeError: 'NoneType' object has no attribute 'values'

…-state

This is more clean than using a price as id.

Add new columns "virtual" and "custom" to Orders table. "Virtual" is for
indicating virtual order, "Custom" is for adding any custom data in
string format, may be used by any strategy to add additional info about
order.

This commit also introduces database migrations mechanism via alembic
tool. Seems like it's the best solution for sqlalchemy to handle schema
updates.

Use new storage methods to save/restore virtual orders

Add notice, add workaround to purge test worker orders from the db on
teradown.

New method is needed to be able selectively remove orders from the db.

Closes: Codaone#636

* Operational depth following is strict now (don't keep additional 5
  orders on price movements)
* Order size check is removed as orders are kept in state and not
  restored from scratch
* Order replacement is now happening in single `for` cycle instead of
  one-per-maintenance