Unit test connection and connection restoration after failure

[krassowski]

Follow up on #4. Two major scenarios to test:

• LSP server not available when notebook starting, but available afterwards
• connection to the LSP server temporarily lost

Also:

• the LSP server encounters a serious error and the pipe gets broken; it should be restarted; we should require that no more that 3 failures occur or that they occur not more often than once every 15 minutes.