WIP: Make multi-session debugging sort of work

This passes the session id around everywhere and ensures that things
like buffer names are all unique.

We now have the _vimspector_session update to point to the active (or
last accessed tab). This feels fairly natural and mostly seems to work.

NOTE: in vscode there is no "multiple tabs" - they actually add the
subprocesses to the stack trace somehow and when you click on a frame in
that it switches to the session for that process. Each process PC is
visible in the editor. It's kind of confusing.

Things still broken:
 - vimspector_session_windows
 - breakpoints need to be project-wide
 - PC display (how to show "all" PCs, or just show the current one for
   the current tab ?)
 - it would be nice for the tterminal buffers to be visible on all tabs.
   not sure how to do that.

plugin/vimspector.vim

@@ -141,6 +141,7 @@ augroup END
 augroup Vimspector
   autocmd!
   autocmd BufNew * call vimspector#OnBufferCreated( expand( '<afile>' ) )
+  autocmd TabEnter * call vimspector#internal#state#OnTabEnter()
 augroup END
 
 " boilerplate {{{