Proposal Related Fixes GRPH-60

[bobinson]

There are handful of security and stability related issues wrt Proposals.

bitshares/bitshares-core#1257

bitshares/bitshares-core#1479

https://www.bitshares.foundation/announcements/2018-12-21-proposal-incident

We need to upgrade the Peerplays blockchain with similar fixes.

[sierra19XX]

There can be a Proposal created many levels of depth.

To prevent this while pushing a proposal, Bitshares check for the level of recursion inside Proposal operations. If it is > 2 * (No. of Witness nodes), an exception will be thrown.

From the Peerplays code I’ve investigated, there is no such check to stop processing a deeply nested proposal.

I feel there are two ways to introduce this in Peerplays,

1. The way Bitshares are doing i.e. during push_proposal we check the level/depth of recursion required.

2. (Another way) Is it a better approach to evaluate a proposal depth at the creation of the Proposal itself?

[bobinson]

@oxarbitrage - Do you have comments about above discussion ?

[bobinson]

PR : #85