Fixed: Use only one file descriptor for each Msgstore
Fixed: Use only one file descriptor for each Msgstore

A MsgStore has 5 files. We were keeping each file opened, therefore
using 5 FDs per MsgStore (that is, per channel). That was wasted
since only one was used at a time.

nats-io/nats-streaming-server,petemiron/nats-streaming-server,nats-io/nats-streaming-server,petemiron/nats-streaming-server,petemiron/nats-streaming-server