Have DAP handle multiple breakpoints at same location

Have DAP handle multiple breakpoints at same location

A user pointed out that if multiple breakpoints are set at the same
spot, in DAP mode, then changing the breakpoints won't reset all of
them.

The problem here is that the breakpoint map only stores a single
breakpoint, so if two breakpoints have the same key, only one will be
stored.  Then, when breakpoints are changed, the "missing" breakpoint
will not be deleted.

The fix is to change the map to store a list of breakpoints.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33467
Reviewed-By: Ciaran Woodward <ciaranwoodward@xmos.com>