Transaction expiration calculation -> change times to only block time

[kristjank]

Is your feature request related to a problem? Please describe.
Now it is possible to set transaction expiration in two ways (block-time and also normal time-to-live before expiration).

Describe the solution you'd like
The only true and measurable time is block height and so in this case also transaction expiration should be set, in explicit value of block height when it should expires.transaction: {

 expiration: 5980000
}

For usability the client applications would still calculate this in a time-spam, so we can show more friendly information to the user sending the transaction. The clients can also suggest like 10 blocks, 2 rounds and then the explicit block expiration height is calculated in the background.

[kristjank]

@vasild did you manage to check into this

[vasild]

No, but I have a rough idea:

1. Rename transaction.expiration -> transaction.expirationSecondsSinceGenesis
2. Introduce new transaction.expirationHeight
   If 2. is set then override/ignore 1. and expire based on blocks, if 2. is not set then do as the current code does (expire by seconds).
3. Get the user-facing apps to set 2.
4. Delete 1. from the code as it will be unused after 3.

IMO the user-facing apps should present the user time-based expiration dialogs, like "expire after N hours approximately" or "expire at 14:56 on Nov 1 2018 approximately" and convert into block number under the hood. Then maybe have some advanced, geeks-only option to set/view expiration block number directly.