is_member() requests all members to know if an identity is member will explose
[`is_member()`](https://git.duniter.org/clients/python/silkaj/blob/39f9bb9cc938ab12aa46b1a5794200725fd1641f/silkaj/wot.py#L192-197) is currently retrieving the whole list of members from `/wot/members` to know if an identity is a member or not.
The more members it will be, the more this request will slows down silkaj to determine if an identity is member or not.
The usage of `/wot/identity-of/<id/pubkey>` says if an identity is member or not when sending error code.
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### Todo
- [x] remove not used `get_pubkey_from_uid()`
- [x] remove `get_uid_from_pubkey()`
- [x] 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.
- [ ] In case the identity is not member, try with `/wot/lookup/*` to know the corresponding uid/pubkey?
- [x] change `is_member()` as a wrapper of `identity_of()`
- [x] change usage in `id`, `cert`, `tx`, `net`, and `blocks`
If Silkaj requests several time at the same moment, we get a `429 Too Many Requests`. #206 should solves this.
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GVA API should allow one request for all needed identities.
issue