.tell db should use primary key based on time

[ctian1]

If someone .tells someone the same message multiple times, all of them will be marked as read when the other person chats.

For example:

<1> .tell 2 hi
<1> .tell 2 hi
<1> .tell 2 hi
<1> .tell 2 hi
<2> blah
-Notice from CloudBot- You have a message from 1: hi (+3 others)
<2> .showtells
-Notice from CloudBot- You have no tells.

This is because the plugin deletes with the message, chan, and server instead of using a primary key including time.

[dmptrluke]

Yup. Agreed. This is also needed to add MySQL support because MySQL has a primary key length limit