Refactor msg_server_submit_txs.go

[sampocs]

Background

There's room for improvement to make msg_server_submit_txs.go more readable.

Suggested Design

• Rename file to submit_ica_txs
• Remove SubmitTxByDayEpoch and SubmitTxByStrideEpoch
• Create IcaTx struct
• Separate building of IcaTx struct from function that calls the Ica (and add test coverage that evaluates the individual ica messages)

[ThanhNhann]

OMG I'm writing tests for this function, any logic will be changed ser?