[feat] connection pool info API

[JimmyDaddy]

getConnectionSize()/getPoolSize() ?
getAvailableConnections() ?
getWaitingConnections() ?

[cyjake]

I'd prefer adhering to existing pool api of mysqljs/mysql with few options like initialSize and idleTimeout added. To implement a customized pool layer isn't very complicated, I think it's time for us to start.

[cyjake]

For example, the proposed api in issue description can mostly be replaced with

proposed	possible existence
getConnectionSize()/getPoolSize()	pool.options.connectionLimit
getAvailableConnections()	pool.connections.length
getWaitingConnections()	N/A

Does getWaitingConnections() mean idle connections? If so we need two connections arrays, idleConnections and connections.

[JimmyDaddy]

getAvailableConnections() means idle connections, getWaitingConnections() means waiting connections, in Sequelize is waitingClientsCount, in pg those three readonly variables' names are properly:

 readonly totalCount: number;
 readonly idleCount: number;
 readonly waitingCount: number;

[cyjake]

waitingClientsCount or waitingCount does not mean waiting connections, which rather mean waiting queries that are postponed while all connections are checked out.