Add downloader hash, flush and complete metrics

[mh0lt]

This adds metrics to indicate the downloaders internal processing for:

Flush - the amount of data currently flushed to disk
Hash - the amount of data currently hashed
Complete - the amount of data currently completed

For logging these are translated to rates in the same way that downloading and uploading are:

 progress="21.15% 261.7GB/1.2TB" time-left=1hrs:48m total-time=22m0s download=516.1MB/s flush=120.6MB/s hash=247.5MB/s complete=153.0MB/s upload=0B/s peers=49 files=527 metadata=455/527 connections=50 alloc=16.0GB sys=18.4GB

This is useful for 2 things:

1. To get an overall idea about the overall flow
2. At the end of the download typically the download finished before the other stages, now rather than seemingly sitting idle it is possible to see what the down loader is doing.

If all 4 metrics are 0 then the downloader has stalled.

The torrent lib update also includes a fix which should remove this http.transport panic:

panic: net/http: internal error: connCount underflow

goroutine 32675 [running]:
net/http.(*Transport).decConnsPerHost(0xc003a3a000, {{0x0, 0x0}, {0xc000f1e600, 0x5}, {0xc01a014e70, 0x2f}, 0x0})
        net/http/transport.go:1511 +0x405
net/http.(*persistConn).closeLocked(0xc171c378c0, {0x32163c0, 0x479f9a0})
        net/http/transport.go:2764 +0xe5
net/http.(*persistConn).close(0xc003a3a000?, {0x32163c0?, 0x479f9a0?})
        net/http/transport.go:2754 +0xa8
net/http.(*Transport).putOrCloseIdleConn(0xc003a3a000?, 0xc171c378c0)
        net/http/transport.go:913 +0x2d
net/http.(*Transport).dialConnFor(0xc003a3a000, 0xc05af76420)
        net/http/transport.go:1491 +0x11d
created by net/http.(*Transport).queueForDial in goroutine 30586
        net/http/transport.go:1461 +0x1cd