Add unique constraint on civicrm_entity_financial_account

There is no constraint on civicrm_entity_financial_account to ensure that there is only a single unique value for its natural key. If there are duplicates, then which of the non-unique values is returned is non-determinate, and accounting becomes indeterminate...ie BAD things happen.

On upgrade, run the following:

ALTER TABLE civicrm_entity_financial_account ADD CONSTRAINT civicrm_entity_financial_account_natural_key UNIQUE (entity_table, entity_id, account_relationship);

Modify https://github.com/civicrm/civicrm-core/blob/master/xml/schema/Financial/EntityFinancialAccount.xml to add this index to tarball.