Allow witnesses / committee members / workers to disable themselves

From witness channel on telegram:
> We should ask the devs to incorporate the disable feature for witnesses.

> Plus deactivation for committee, and workers.

Sometimes, witnesses or committee members suddenly get voted in who have previously declared that they step down / resign from their posts. In the case of witnesses this leads to instability of the chain, in the case of committee members it can make it difficult to enact important governance decisions. It should therefore be possible for witnesses and committee members to flag their resignment on-chain, so that they are ignored during voting. Also, a witness who has missed all his blocks between two maintenance intervals should be disabled automatically.

A comparable mechanism for witness resignment already exists on Steem on other Steem-based chains.

Similarly, it should be possible to resign workers. A typical use-case might be that the funding goal has been reached prematurely. Resigning such a worker would free up the budget for other projects.

The resignment must be made reversible.

Start by reviewing the requested witness, committee-member, and worker resignation behavior, then compare the existing witness-resignation mechanisms on Steem and other Steem-based chains mentioned in the issue. Define reversible on-chain disabling, automatic witness disabling after missed blocks, and budget release for resigned workers; the proposal should specify how each behavior affects voting and funding.

Written by the indexing model from the issue text.