Evaluate Envelope-related policy constraints

[AnnaShaleva]

Rationale

#413 and #422 introduce MaxEnvelopesPerBlock, MaxEnvelopeGasLimit, EnvelopeFee. We need to make careful evaluations of these constraints to use them on mainnet/testnet so that it's not possible for the users to occupy all block space by envelopes or decrypted transactions. Ref. #422 (comment).

Implementation

Define values, set them in Policy contract for both Mainnet and Testnet.

[txhsl]

For privnet and testnet, I've set MaxEnvelopesPerBlock to 10, MaxEnvelopeGasLimit to 2,000,000, and leave EnvelopeFee as 0.

[AnnaShaleva]

sharePeriodDuration will be 2880 blocks (about 4 hours), MaxEnvelopesPerBlock will be 10, MaxEnvelopeGasLimit will be 2000000.

Done by @txhsl.

@txhsl we need to set EnvelopeFee to some non-zero value in testnet, it can't be 0.

[txhsl]

we need to set EnvelopeFee to some non-zero value in testnet, it can't be 0.

Yes, the fee is important. But I suggest that we leave the decision to the marketing team.

1. A 0 fee is possible since we restricted the envelope number per block, so the performance effect should be quite slight;
2. I've received some advice for 0 fee from marketing, although it's personal.

I'm waiting for their response. They are good with this.

[txhsl]

I'm going to copy the same policy constraints from Testnet to Mainnet:

• MaxEnvelopesPerBlock = 10;
• MaxEnvelopeGasLimit = 2000000;
• EnvelopeFee = 0.

Also, will use the same sharePeriodDuration = 2880.

I haven't received any feedback about these variables, from users or developers, maybe nobody is using them. So for now, let's have them the same on both of our networks.

Further decisions can refer to EIP-7825: Transaction Gas Limit Cap and miner: default gaslimit 45M.