Add config option to limit history size

[maxpoulin64]

This adds a (temporary?) config option to limit the amount of messages stored per channel to avoid the server's memory usage to grow as channels fills up with messages.

• It defaults to 0 so the regular behavior of infinite history is kept, but users can set a size limit to avoid their server from running out of memory over time.
• It just splices out the older messages from the array, first in first out.
• I don't know how efficient node is at handling splicing, but it might possibly be expensive if the number of history lines is high enough.

[floogulinc]

There is something similar in #391 that also can load from the log file if enabled.