pollable_fd_state: rename speculate_epoll and take_speculation.

[balusch]

• speculate_epoll is not only used in epoll backend, but also in other backends, the name of which does not reflect the reality. Here we rename it to speculate.
• take_speculation sounds like to do or perform speculation, but what we actually do is to invalidate the speculation. Here we rename it to check_and_revoke_speculation.
• update the comment of events_known.

Signed-off-by: Jianyong Chen [email protected]

[balusch]

@avikivity Hi, Avi, could you please have a look again?