Remove pkg satellite/payments/monetary as it moved to storj.io/common.
Update all code pkg references from monetary to common/currency.
Change-Id: If2519f4c80cf315a9299e6521a6b9bbc6c399156
Why: big.Float is not an ideal type for dealing with monetary amounts,
because no matter how high the precision, some non-integer decimal
values can not be represented exactly in base-2 floating point. Also,
storing gob-encoded big.Float values in the database makes it very hard
to use those values in meaningful queries, making it difficult to do
any sort of analysis on billing.
For better accuracy, then, we can just represent monetary values as
integers (in whatever base units are appropriate for the currency). For
example, STORJ tokens or Bitcoins can not be split into pieces smaller
than 10^-8, so we can store amounts of STORJ or BTC with precision
simply by moving the decimal point 8 digits to the right. For USD values
(assuming we don't want to deal with fractional cents), we can move the
decimal point 2 digits to the right.
To make it easier and less error-prone to deal with the math involved, I
introduce here a new type, monetary.Amount, instances of which have an
associated value _and_ a currency.
Change-Id: I03395d52f0e2473cf301361f6033722b54640265
