x/lockup: Ensure events are emitted and tests added

[stackman27]

Background

Currently, our tests are missing assertions about events. Moreover, it seems that some of our events are missing based on #1939

The lockup module emits the following events:

There are 4 types of events that exist in lockup module:

* `types.TypeEvtLockTokens` - "lock_tokens"
* `types.TypeEvtAddTokensToLock` - "add_tokens_to_lock"
* `types.TypeEvtBeginUnlockAll` - "begin_unlock_all"
* `types.TypeEvtBeginUnlock` - "begin_unlock"

### `types.TypeEvtLockTokens`

* `types.AttributePeriodLockID`
* `types.AttributePeriodLockOwner`
* `types.AttributePeriodLockAmount`
* `types.AttributePeriodLockDuration`
* `types.AttributePeriodLockUnlockTime`

### `types.TypeEvtAddTokensToLock`

* `types.AttributePeriodLockID` 
* `types.AttributePeriodLockOwner`
* `types.AttributePeriodLockAmount`

### `types.TypeEvtBeginUnlockAll`

* `types.AttributePeriodLockOwner`
* `types.AttributeUnlockedCoins`

### `types.TypeEvtBeginUnlock`

* `types.AttributePeriodLockID`
* `types.AttributePeriodLockOwner`
* `types.AttributePeriodLockDuration`
* `types.AttributePeriodLockUnlockTime`

Therefore, this issue is twofold:

• Investigate if / which events are missing and restore them

• Add assertions in tests that events are being emitted in the correct places

Suggested Design

Very similar to what @p0mvn suggested for superfluid events! #2194

Acceptance Criteria

• lockup events are emitted correctly in the right locations
• tests added for attributes
• tests added for verifying that events are emitted in the correct locations
• every event is documented in lockup README
  • what the events is for / when it is emitted
  • what attributes it contains
• final design is consistent with x/gamm and x/superfluid events

[stackman27]

Epic #2195

[ValarDragon]

See PR discussion