Inconsistency between constant/variable names

[hallazzang]

As a result of #376, we have converted Id to ID except for proto-related fields.
But as x/liquidity/types/events.go has nolint flag, the file was not covered by the conversion:

liquidity/x/liquidity/types/keys.go

Line 26 in 2a38c43

GlobalLiquidityPoolIDKey = []byte("globalLiquidityPoolId")

liquidity/x/liquidity/types/events.go

Line 14 in 2a38c43

AttributeValuePoolId = "pool_id"

To keep the convention consistent, and as far as we decided to use Id for proto files, it'd be better to rollback the change and use Id instead of ID except for function-scoped variables.

[hallazzang]

While this is an insignificant issue and those constant/variables are used internally, we're not gonna revert changes made in #376.