Sync stalling - low transaction rate

[fireduck64]

Something seems to go wrong in the initial block processing. I'll see it hum along at 400 transactions per second and then something will happen and it will go to 0 to 20 tps. The only thing I can do to get it out of this state seems to be to ctrl+c it and restart it.

Not sure what is going on. When this happens, all the threads seem to be waiting on network read deep in the mongo client.

[fireduck64]

This was running out of memory due to a huge queue of blocks building up in bitcoinj.

I've made a change to bitcoinj to avoid this, including this file:
https://github.com/fireduck64/jelectrum/blob/master/src/Threading.java