Add support for SendBatchToTarget

[Yu-Xie]

This is a re-write of #576 after addressing feedbacks and rebasing.

Related issue: #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.

[d-belov]

Hello there :)

Our team currently faces similar scalability challenges and we would like to use this batching solution as well.

@ackleymi, it would be great if this PR could be merged (considering it looked good enough back then) as it would allow us to avoid forking the library.

Would this be possible at all? We would be happy to contribute.