fix(pool): synchronously release pool connection

Rapidly-fired queries can exhaust the pool capability and lead to odd
results. In test, we've witnessed "Connection Terminated" flakiness
with many fast test suites. This, combined with setting
`lazyConnect: true` and `idleTimeoutMillis: 0`, alleviates the issue.

Author: Samuel Reed

lib/postgresql.js

@@ -40,9 +40,7 @@ exports.initialize = function initializeDataSource(dataSource, callback) {
 
   if (callback) {
     if (dbSettings.lazyConnect) {
-      process.nextTick(function() {
-        callback();
-      });
+      process.nextTick(callback);
     } else {
       dataSource.connecting = true;
       dataSource.connector.connect(callback);
@@ -97,9 +95,9 @@ PostgreSQL.prototype.getDefaultSchemaName = function() {
  */
 PostgreSQL.prototype.connect = function(callback) {
   const self = this;
-  self.pg.connect(function(err, client, done) {
+  self.pg.connect(function(err, client, releaseCb) {
     self.client = client;
-    process.nextTick(done);
+    process.nextTick(releaseCb);
     callback && callback(err, client);
   });
 };
@@ -123,17 +121,13 @@ PostgreSQL.prototype.executeSQL = function(sql, params, options, callback) {
     debug('SQL: %s', sql);
   }
 
-  function executeWithConnection(connection, done) {
+  function executeWithConnection(connection, releaseCb) {
     connection.query(sql, params, function(err, data) {
       // if(err) console.error(err);
       if (err) debug(err);
       if (data) debugData('%j', data);
-      if (done) {
-        process.nextTick(function() {
-          // Release the connection in next tick
-          done(err);
-        });
-      }
+      // Release the connection back to the pool.
+      if (releaseCb) releaseCb(err);
       let result = null;
       if (data) {
         switch (data.command) {
@@ -169,9 +163,9 @@ PostgreSQL.prototype.executeSQL = function(sql, params, options, callback) {
     // Do not release the connection
     executeWithConnection(transaction.connection, null);
   } else {
-    self.pg.connect(function(err, connection, done) {
+    self.pg.connect(function(err, connection, releaseCb) {
       if (err) return callback(err);
-      executeWithConnection(connection, done);
+      executeWithConnection(connection, releaseCb);
     });
   }
 };