Add support for SendBatchToTarget

[Yu-Xie]

This is a re-write of https://github.com/quickfixgo/quickfix/pull/576 after addressing feedbacks and rebasing.

Related issue: https://github.com/quickfixgo/quickfix/issues/555

[Yu-Xie]

@Yu-Xie if you would change SendAppToTarget to SendBatchToTarget and SaveMessagesAndIncrNextSenderMsgSeqNum to SaveBatchAndIncrNextSenderMsgSeqNum I will merge. I'd like to keep func names immediately obvious to prevent accidental misuse. Also if the FileStore and MongoStore impls are easy enough to implement for you, please do so.

@ackleymi This addresses your feedbacks (renaming & implementing all the stores) -- Please take a look.