Recognize fatal mysql2 connection errors

Connections created by MySQL2 driver transition to broken state when
network errors or timeouts occur. This condition is not visible to the
connection pool in Knex.

If Knex detects a timeout and tries to close a connection, this causes
an uncaught exception from MySQL2. Similarly the connection pool never
realizes the connection is in a broken state, and thus will never
replace it.

Unfortunately the MySQL2 connection does not expose a state, but it can
be inferred by checking for `connection._fatalError`.

Fixes knex#1853

src/dialects/mysql2/index.js

@@ -29,8 +29,8 @@ assign(Client_MySQL2.prototype, {
     return require('mysql2')
   },
 
-  validateConnection() {
-    return true
+  validateConnection(connection) {
+    return !connection._fatalError
   },
 
   // Get a raw connection, called by the `pool` whenever a new
@@ -50,6 +50,10 @@ assign(Client_MySQL2.prototype, {
     })
   },
 
+  destroyRawConnection(connection) {
+    if (!connection._fatalError) return Client_MySQL.prototype.destroyRawConnection(connection)
+  },
+
   processResponse(obj, runner) {
     const { response } = obj
     const { method } = obj