Tested by transferring 29GiB data, around 246569 files
over 100Gig network.
- master branch
```
time mc mirror /root myminio/testbucket/
real 2m10s
```
- after changes
```
time mc mirror /root myminio/testbucket/
real 1m2s
```
By default runtime.NumCPU() workers are started.
This PR removes --parallel flag and implements a dynamic way of adding
threads to accelerate mirror/cp command.
After the calculation of objects that need to be copied/removed, mc
sends tasks (upload/remove) to a parallel manager which monitors the
progress of tasks and decide to allocate more threads to it to
accelerate the overall work.
This is very useful when we copy/mirror many relatively small files.
The probe package is no longer available in the Minio server
repo. This change just moves probe from the vendor directory to its
own dir under `pkg`.
This is a maintenance fix.
