New p2p-peer-limit

• Does not require a new p2p protocol version. Works with existing peers.
• Defaults to 6. Disabled if p2p-accept-transactions=true.
• Of the specified p2p-peer-address, picks the best (p2p-peer-limit) peers to remain connected to.
• All connections that have received a vote first from the connection are not disconnected.
• All connections that have received a block first from the connection over the last 10 minutes are not disconnected.
• The top (p2p-peer-limit) ping times for connections are not disconnected.

The following were added to connection status response:

• start_time - when the connection was established.
• unique_trx_count - number of transactions first received on connection.
• unique_blk_count - number of blocks first received on connection.

Resolves #1155