cbbackupmgr should use '--auto-select-threads' by default

What's the issue?
By default 'cbbackupmgr' will only use a single thread when performing a backup/restore (internally we already do some sort lived/heavy IO tasks concurrently). Using only a single thread can lead to poor performance/low throughput.

We've seen several cases where users haven't supplied '--threads' or have supplied a surprisingly low value for the machine that they're running on.

Considering that backups taken with the backup service/the autonomous operator already supply '--auto-select-threads' it seems sensible to enable by default in 'cbbackupgmr'.

What's the fix?
Have 'cbbackupmgr' automatically determine how many threads to use by default. Note that we should honor the value specified by the user if they are e.g. it should take precedence.