Enhance ModelController arguments

[SYangster]

ModelController enhancements #2049

1. BaseFedAvg: change min_clients -> num_clients for clarity now that we hid wait_time_after_min_received and we are sending to num_clients targets rather than all clients.
2. Improve docstring of sample_clients()
3. Clarify meaning of timeout in send_model_and_wait() and send_model()
4. Add min_responses and hide wait_time_after_min_received (Add min_responses to send_model_and_wait() #2675)

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[chesterxgchen]

LGTM

[SYangster]

/build