lib-smtp: server: Perform output stream error handling in one place.

lib-smtp: server: Remove undefined smtp_server_reply_send_more() from smtp-server-private.h.

lib-smtp: server: Handle output stream errors in a separate function.

lib-smtp: server: Put reply content in a separate struct. This allows duplicate replies to share content for a multi-reply command.

lib-smtp: server: Add RCPT parameters directly in smtp_server_transaction_add_rcpt(). This makes more sense. Before, these were added to the recipient in smtp-server-cmd-rcpt.c, after the recipient was added.

lib-smtp: server: Add APIs for halting and resuming connection command handling. Also adds function to start the connection in a pending (halted) state. This way the greeting can already be sent (over SSL if needed), while deferring command handling until some external activity is completed.

lib-smtp: server: Protect smpt_server_connection_start() from being called multiple times.

lib-smtp: server: transaction: Added smtp_server_transaction_rcpt_count().

lib-smtp: Implemented SMTP server.