simplyfying manager, added go.mod and check for task execution in tests

[chiamakacha]

inspired from: @chr1sto14

1. Simplified manager, no need to fill the channel on initialisation
2. added check in tests to verify if all tasks are being executed.
3. changed to go module structure

[zenthangplus]

Thank @chiamakacha @chr1sto14, i'm working on this MR

[zenthangplus]

@chiamakacha i see you remove close method, why you do that? please explain more

[chiamakacha]

@chiamakacha i see you remove close method, why you do that? please explain more

@zenthangplus in original code, we were creating a go thread for management and having a close method to clean up resources i.e, go thread made sense.
Now we are not using any infinite go thread, so having close method made no sense to me. Even in the tests we had to use close with break statement and if we remove break from the if block, function doesn't behave properly.

[chiamakacha]

One more point:
Lets say we use close method to close manager channel, then wait method will throw runtime error, so user will have to be more cautious using close method.
imo removing close method makes this package simple.