After running plague-server (with the mysqld backend in my case) for a while,
the plague-server quits with the following message:

Error: (1040, 'Too many connections')

Grepping through the logs shows that this seems to happen somewhere in the
depsolver code, as the error appears after a started depsolver or a finished
depsolver action.

The error itself is coming from the mysql server, which has a connection limit
of 100 concurrent connections.

Calling show-processlist on the mysql server gives the following output:

mysql> show processlist;
+----+--------+-------------------------+--------+---------+------+-------+------------------+
| Id | User   | Host                    | db     | Command | Time | State | Info
            |
+----+--------+-------------------------+--------+---------+------+-------+------------------+
|  3 | plague | horscht.bawue.net:49928 | plague | Sleep   |  886 |       | NULL
            |
| 13 | plague | horscht.bawue.net:49949 | plague | Sleep   |   37 |       | NULL
            |
| 20 | plague | horscht.bawue.net:49990 | plague | Sleep   |  746 |       | NULL
            |
| 23 | plague | horscht.bawue.net:49991 | plague | Sleep   |  741 |       | NULL
            |
| 24 | plague | horscht.bawue.net:49992 | plague | Sleep   |  740 |       | NULL
            |
| 25 | plague | horscht.bawue.net:49999 | plague | Sleep   |  735 |       | NULL
            |
| 27 | plague | horscht.bawue.net:50002 | plague | Sleep   |  730 |       | NULL
            |
| 33 | plague | horscht.bawue.net:50006 | plague | Sleep   |  708 |       | NULL
            |
| 34 | plague | horscht.bawue.net:50008 | plague | Sleep   |  703 |       | NULL
            |
| 35 | plague | horscht.bawue.net:50011 | plague | Sleep   |  692 |       | NULL
            |
| 36 | plague | horscht.bawue.net:50012 | plague | Sleep   |  687 |       | NULL
            |
| 55 | plague | horscht.bawue.net:50092 | plague | Sleep   |  528 |       | NULL
            |
| 56 | plague | horscht.bawue.net:50093 | plague | Sleep   |  523 |       | NULL
            |
| 57 | plague | horscht.bawue.net:50097 | plague | Sleep   |  512 |       | NULL
            |
| 58 | plague | horscht.bawue.net:50098 | plague | Sleep   |  507 |       | NULL
            |
| 79 | root   | horscht.bawue.net:50350 | NULL   | Query   |    0 | NULL  | show
processlist |
+----+--------+-------------------------+--------+---------+------+-------+------------------+