fine tune the number of ballot/vote consumers

[JMLX42]

Each ballot/vote consumer will connect to the ethereum node.
It implies using file descriptors (at least for sockets).
Having a lot of consumers might make the ethereum node crash because it won't have any file descriptor left.

When it happens, the geth logs show the following message:

http: Accept error: accept tcp 127.0.0.1:8545: accept: too many open files; retrying in 10ms

To make sure it doesn't happen:

• augment the maximum number of file/socket descriptors for geth
• make sure ballot_consumer_count, vote_consumer_count and the queue prefetch value won't cause a file desriptor shortage

[JMLX42]

Related issue: ethereum/go-ethereum#2358