Ensure that correct data about spending transaction is present in JSON

InputScriptPubKeys from different transactions could mix and corrupt
validation context. PubKeys moved from list to dictionary of lists,
with transaction hash as key.

When node accepts transaction or receives block, it tries to load all
inputs to mempool. Trying to load zero-inputs to mempool leads to
corruption - valid transaction may be removed from mempool during
conflicts check. To prevent this, inputs selection algorithms modified
to ignore zero-inputs. Conflicts check ignores them too.