fix: truly stop lsp client and handle rust-analyzer with rustaceanvim.

[ayamir]

This patch adds two changes:

1. Use client.stop() to stop client and truly save memory. Current impl doesn't work for me, the lsp server still occupies lots of memory in the background.
2. Start the rust-analyzer managed by rustaceanvim correctly.

[Zeioth]

This is not gonna be merged as:

• Rustacean is a plugin, not a lsp server.
• The PR modifies unnecessary lines.

Thank you for sending it. It might serve as reference for other people with your same case of use.
Closing.