Simplify MLLP.Sender/Client api and standardize errors

[starbelly]

• reduce the various send_* functions to send/2,/3,/4 and send_async/2,3
• add auto_reconnect_interval option
• wrap and unwrap payloads by default
• only verify a response with MLLP.Ack if the payload provided is an HL7.Message.t()
• move the initial connection attempt into a handle_continue/2 clause
• normalize (validate) host parameter to a tuple and raise if invalid
• Always log an error when we fail to establish a connection
• Store connection failures in state so we can return the caller the last known error in case they try to send a message while the connection is still being established
• Add and standardize on MLLP.Client.Error (exception) for returned errors
• Use Config vs Mix.Config in config/*
• misc refactors, enhancements, and fixups

[vikas15bhardwaj]

Some minor comments:

• Can we add send_async to ClientContract
• We should change :sender to client in one telemetry event on line 366 in client module
• In multiple tests, we are using sender as variable names for client, should we change those?