`pending` -> `receivable`, `pending_exists` -> `receivable_exists`

[keeri]

these two need updating, as pending is depreciated ( https://docs.nano.org/commands/rpc-protocol/#receivable )

i'd do it myself but there are plenty references and i might miss a few

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note: accounts_receivable does NOT exist yet, and is still called accounts_pending. it should be renamed in future releases