Various minor changes that help alleviate annoyances

Needs test-tools!158 (merged) and schemas!58 (merged)

This MR deals with the following:

1. corrections to help with unwanted differences between state of hived and block explorer:

• to_withdraw/withdrawn fields of account_object are cleared after power down
• preparation for fix of artificial 1 in power down rate despite account not doing power down (the preparation will activate on HF28, after that we can apply actual fix similar to !1017)

2. corrections in mechanism for detecting private key leak in memo:

• added missing check in witness plugin for memo of recurrent_transfer_operation
• consolidated collection of potential leaked keys for witness plugin and wallet, so new key candidates for one are also used for the other
• supplemented check with key based on <account>+"memo"+<content of memo>
• supplemented check with direct WIF form of private key

3. corrections for problem of witness plugin accessing comment before it was created (issue #565 (closed)):

• moved soft limit on beneficiaries from witness plugin to main code
• moved extra soft limit on comment depth from witness plugin to main code

4. removed limit on vote edits - will activate for HF28 (issue #458 (closed))