HD key paths used for each input to derive a signing key. It's useful for multisig inputs only.
The keys used to create a single P2SH address have the same derivation path, so to use a correct key each signer
has to know a derivation path of signing keys used by previous signers. For each input signers will use the
same derivation path and we need to store only one key path per input. As TransactionInput is mutable, inputs
are identified by their scriptPubKeys (keys in this map).