now only converting the signature to an object once, and not everytime within the public key loop
now only converting the signature to an object once, and not everytime within the public key loop

Summary: Improved the performance of the input signature verification by moving a loop-independent piece of code outside of a loop.

Reviewers: ben, benchan

Reviewed By: ben, benchan

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D1460

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS