Add compression ratio to pbm-speed-test

At the time of completing PBM-447 we cannot get byte size uploaded from the IOWriter streams, so we couldn't calculate and print the compression ratio as well as speed.

After the underlying issue of receiving byte size transferred at the end of a transfer is solved let's add it where the "HERE" is shown below.

akira:percona-backup-mongodb$ bin/pbm-speed-test compression --compression=s2 --size-gb 10
Test started ..
10.00GB sent in 5s.
Avg upload rate = 2006.55MB/s.
### HERE ###