(core) Fixing lock issues and reverting back to single connection.

Summary:
Removing `createNewConnection` method that was used in tests to create a
"scoped" version of `HomeDbManager`. Currently this won't work as there are
many methods (like `Users.findOne`) that are using the default (global) connection.

Additionally `HomeDBManger` had couple of bugs that were causing locks, which
manifested themselves in postgresql tests (that are not serializing transactions).
Repository methods like `Users.findOne` or `user.save()`, even when wrapped in
transaction were using a separate connection from the pool (and a separate
transaction).

Some tests in `UsersManager` are still skipped or refactored, as sinon's `fakeTimers`
doesn't work well with postgresql driver (which is using `setTimout` a lot).

Date mappings in `User` entity were fixed, they were using `SQLite` configuration only,
which caused problems with postgresql database.

Test Plan: Refactored.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4342

app/gen-server/entity/User.ts

@@ -26,10 +26,10 @@ export class User extends BaseEntity {
   @Column({name: 'picture', type: String, nullable: true})
   public picture: string | null;
 
-  @Column({name: 'first_login_at', type: Date, nullable: true})
+  @Column({name: 'first_login_at', type: nativeValues.dateTimeType, nullable: true})
   public firstLoginAt: Date | null;
 
-  @Column({name: 'last_connection_at', type: Date, nullable: true})
+  @Column({name: 'last_connection_at', type: nativeValues.dateTimeType, nullable: true})
   public lastConnectionAt: Date | null;
 
   @OneToOne(type => Organization, organization => organization.owner)