Delay LS shutdown when last client disconnects

[hubertp]

Pull Request Description

Rather than closing Language Server immediately, we delay the shutdown until some timeout hits. This gives a chance for new clients to connect without paying the price of the initialization again.

More importantly, during hibernation/restart, the connection between client (IDE) and LS is severed so it appears as if client disconnect. In fact a few moments later IDE would attempt to re-establish the connection on the same port. Without this change, LS shutsdown and further attempts to connect on that particular port will fail.

There are still problems on the IDE-side after waking up from hibernation but it is not related to Language Server.

Important Notes

Partially solves #5197.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.

[hubertp]

Ping @4e6 @JaroslavTulach