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

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