Increase LineOnlyReader.MAX_LENGTH to prevent large gentxs from making mining proxy disconnect

[forrestv]

Fixes problem with connecting to P2Pool:

2013-05-26 17:01:44,371 INFO stats stats.print_stats # 1 peers connected, state changed 1 times
2013-05-26 17:01:44,372 INFO proxy mining_proxy.on_connect # Connected to Stratum pool at 127.0.0.1:9332
2013-05-26 17:01:44,372 INFO proxy mining_proxy.on_connect # Subscribing for mining jobs
2013-05-26 17:01:44,396 INFO proxy client_service.handle_event # Setting new difficulty: 0.999984741211
2013-05-26 17:01:44,396 INFO proxy mining_proxy.on_disconnect # Disconnected from Stratum pool at 127.0.0.1:9332
2013-05-26 17:01:44,396 INFO stats stats.print_stats # 0 peers connected, state changed 1 times

[slush0]

How long are usually such transactions? 1.6 MB seems too much, considering trivial DoS attack on server's RAM using thousands of concurrent connections...

[slush0]

I cannot merge this patch to generic implementation, it has too dangerous consequences. Maybe the right solution is to have such option in server settings...?