Add DDP.onReconnect(), deprecate conn.onReconnect

[hwillson]

Creating a new PR to continue work on #5677 (since the original PR's branch is locked). Original PR description:

Deprecate use of connection.onReconnect = func. Instead, a new
DDP.onReconnect(callback) method should be used to register callbacks to call
when a connection reconnects. The connection that is reconnecting is passed as
the only argument to callback. This is used by the accounts system to relogin
on reconnects without interfering with other code which uses
connection.onReconnect.

Definitely still work in progress.

Fixes #5665.

[hwillson]

Hi all - we should be good to go here (ready for review), so I'll remove the WIP prefix. If anyone is interested in a quick overview of this issue and the fix, see the updated History.md entry. A few extra things to note:

1. These changes are backwards compatible (even though the existing connection.onReconnect handling is mostly-broken, people still might be using it as is; for a good breakdown of why it makes sense to preserve this broken backwards compatibility, see User logging in or out overwrites onReconnect and code using onReconnect breaks resume-on-reconnect #5665 (comment)).

2. The DDP.connect docs will have to updated after (if) this PR is merged. The onReconnect section under DDP.connect should be removed, and a new DDP.onReconnect section should be added. I'll prep a docs PR for this.

Thanks!