use memory for async events when necessary and dequeue them as
needed, for all synchronous events customers must enable
```
MINIO_API_SYNC_EVENTS=on
```
Async events can be lost but is upto to the admin to
decide what they want, we will not create run-away number
of goroutines per event instead we will queue them properly.
Currently the max async workers is set to runtime.GOMAXPROCS(0)
which is more than sufficient in general, but it can be made
configurable in future but may not be needed.
With the current asynchronous behaviour in sending notification events
to the targets, we can't provide guaranteed delivery as the systems
might go for restarts.
For such event-driven use-cases, we can provide an option to enable
synchronous events where the APIs wait until the event is successfully
sent or persisted.
This commit adds 'MINIO_API_SYNC_EVENTS' env which when set to 'on'
will enable sending/persisting events to targets synchronously.
This is to ensure that there are no projects
that try to import `minio/minio/pkg` into
their own repo. Any such common packages should
go to `https://github.com/minio/pkg`
