Fix small bugs in the async client

[ekouts]

Fixes:

• After the rate limit for a microservice is reached, the next request should be scheduled after the Retry-After header and the user-imposed rate limit. Both should be respected so we keep the larger value.
• Change bytes and lines to str (instead of int) in the head and tail commands in the async client, to follow the sync client. In the sync client this was already fixed.
• Add skip_ending and skip_beginning in head and tail commands respectively, in the async client.
• Changed the order of bytesand lines arguments in the documentation of head and tail commands, to follow the order of the arguments in the function.

[jpdorsch]

LGTM