Refactor Layer2 Issuance and Vesting Logic

[kmlbgn]

Description

Current logic regarding issuance and vesting involve LP tokens since DP tokens are never minted.

Expected Bahaviour

A refactor is needed in accordance with #675

• Minted LP tokens are only concerned about the drip parameter of dapp.IssuanceConfig. After proposal approval, LPs are automatically sent to Spending Pool with deterministic name lp_appname and rate calculated using drip param.
• premint and postmint parameters are amount of dp/denom token (and NOT LP tokens) to be minted additionally to the dapp.LiquidityPool.pooled_dptoken. They happen at premint_time and postmint_time respectively.
• postmint_time parameter is missing and should be created. It's a duration that starts when premint_time happens.
• premint_time happens when proposal is approved. No change required.
• deposit is an optional parameter of dapp.IssuanceConfig which contains an ALREADY CREATED Spending Pool (must check if spending pool exists otherwise tx must fails with explicit error). If deposit is set, then minted postmint and premint should be sent to the spending pool. If it is not set it should be sent to dapp.teamreserve.

Proto Example

IssuanceConfig

PARAMETER	TYPE
drip	uint64
deposit	string
postmint	types.Int
premint	types.Int
premint_time	uint64
postmint_time	uint64