fix: Disconnect client explicitly

[bmingles]

Client wasn't actually getting disconnected when user clicked disconnect button. I have introduced proper disposal logic to dispose all DH services as part of clearing caches.

Testing

Test 1

• Start DH server
• Connect to server from vscode
• Disconnect via the status bar item
• Should see "Disconnected from Deephaven server: ..." toast message
• Kill server.
• Should not see a new "Disconnected from Deephaven server..." toast message

Test 2

• Start DH server
• Connect to server from vscode
• Kill server
• Should see "Disconnected from Deephaven server..." toast message

resolves #22