[enh] #135, #189, #206: wot requests improvements:

- #135: remove get_uid_from_pubkey()
 Replace two uppers function with identity_of() which requests
/wot/identity-of/<pubkey/uid> and only returns members identities with
their pubkey and uid, which improves performances.
- change is_member() as a wrapper of identity_of()
- change usage in `id`, `cert`, `tx`, `net`, `blocks`

- #189: Handle in a good way wot/identity-of requests exceptions

- #206: Set a sleep for async requests: `blocks` and `net` for leaves