[FIX] trade: set default currency pair issuers to null on reset

due to what appears to be an unused boolean 'keepPair' that
defaulted to false, issuers of the currency pair were being
set to the id.account_id, resulting in subscriptions to the
wrong order books.
[FIX] save currency pair whenever settings are updated

the currency pair was not saved when the 'flip' button
was used, or when an existing order was selected
[FIX] balances: get estimate if balances were already loaded

RT-2338
[FIX] make orderbooks subscribe as expected
Merge pull request #1551 from shekenahglory/develop

[FIX] trade: bugs on currency pairs and order books