Start the session only when user starts a conversation

Describe the bug Running chatwoot with a web-integration causes a contact and conversation to be created for each visitor to a website, even if no conversation is ever initiated. This will pollute the database unnecessarily. For example, right now I have three "contacts" shown in my web UI, but when I check the database, I have 4k rows in the contacts table. This seems unnecessarily wasteful. I'd expect that either some process could easily expunge the unused records, or (even better) the rows in the database wouldn't even be created at all unless the user actually initiates some conversation! To Reproduce Steps to reproduce the behavior: 1. Enable web-chat 2. Do nothing, just check your contacts tab and your database rows after some amount of web traffic Expected behavior The number of contacts actually created and the rows in the database should be the same. Screenshots <img width="1488" alt="Screen Shot 2021-03-21 at 9…