Sometimes execution will be halted at a time when there is no current
goroutine (e.g. the process is sleeping). Default the current goroutine
indicator so that trying to prepend the current goroutine to the list
does not cause an error.

Always halt on connect so that :GoDebugConnect will halt the
application and set up breakpoints even when delve was started with
--continue.

It is safe to halt multiple times, so it won't matter what state the
debugger is in when halt is called.

Change function names for readability so that it is easier to reason
about what the code is doing.
* start_cb() -> create_layout()
* stack_cb() -> update_windows()

Rename s:list_breakpoints to s:list_breakpointsigns to disambiguate
whether the breakpoints as understood by the debugger, dlv, or or the
signs placed in the editor are returned.

Synchronize breakpoints whenever connecting to the debugger in case it
was started in multiclient mode and already has breakpoints set.