Use of mgo sessions may replay with more connections than recorded

• A-B: recorded workload does some inserts on 8 connections and then a collection scan on a single connection
• C-D: playback creates up to 45 connections.

However mongoreplay reported using only 11 connections:

2016/10/19 15:01:10 Doing playback at 1.00x speed

2016/10/19 15:01:10 Preprocessing file

2016/10/19 15:01:11 Preprocess complete

2016/10/19 15:02:02 8150 ops played back in 51.156882773s seconds over 11 connections

2016/10/19 15:02:02 Doing playback at 1.00x speed

2016/10/19 15:02:02 Preprocessing file

2016/10/19 15:02:02 Preprocess complete

2016/10/19 15:02:52 8150 ops played back in 49.503066395s seconds over 11 connections

About half of the 45 connections were created during the second spurious playback (TOOLS-1497) so presumably fixing that would cut the number of connections by about half, but this still leaves mongoreplay using 2-3x as many connections as the original workload. This is potentially an issue for workloads with a large number of connections.