bgpd: Refine restarter operation - R-bit & F-bit

bgpd: Refine restarter operation - R-bit & F-bit

Introduce BGP-wide flags to denote if BGP has started gracefully
and GR is in progress or not. Use this for setting of the R-bit in
the GR capability, and not a timer which is set for any new
instance creation. Mark graceful restart is complete when the
deferred path selection has been done and route sync with zebra as
well as deferred EOR advertisement has been initiated.

Introduce a function to check on F-bit setting rather than just
base it on configuration.

Subsequent commits will extend these functionalities.

Signed-off-by: Vivek Venkatraman <vivek@nvidia.com>