bgpd: Intentionally stop some timers on instance removal

bgpd: Intentionally stop some timers on instance removal

When a bgp instance is stopped, with a `no router bgp..`
make sure any timers associated with the instance are stopped
as well.

This issue was discovered when a customer issued a `no router bgp`
while a maxmed timer was operative.  The max-med timer used the
`struct bgp *` as the passed in value for the thread.  The
thread eventually popped after the cleanup and attempted to use
data off in lala land and crashed

Ticket: CM-21895
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>