Mongodb for Chat history

[Ashes47]

Describe the bug
When using mongodb for chat history. Flowise opens multiple connection and does not close them.

Due to this i have hit now 500+ connection limit on mongoDB.

To Reproduce
Steps to reproduce the behavior:

1. Create any agent with mongodb for memory.
2. As you use it you will see number of connections increases for every chat but doesn't come down.

Expected behavior
I think the ideal behaviour is open one connection and keep resuing it, or atleast close all opened connection.

Screenshots

[Ashes47]

#1627.
Creating a singleton class for mondodb instance.