bgpd: Handle route-refresh request received before EoR

bgpd: Handle route-refresh request received before EoR

See the BGP message sequence:

    R1                  R2
    |      updates      |
    |------------------>|
    |                   |
    |  refresh request  |
    x<------------------|
    |                   |
    |   updates cont.   |
    |------------------>|
    |                   |
    |    end-of-rib     |
    |------------------>|
    |                   |

When R1 and R2 establish BGP session, R1 begins to send initial updates.
If R2 sends a route-refresh request before EoR, it's silently ignored
by R1, and routes received earlier have no chance to be processed again.

RFC7313 says, "for a BGP speaker that supports the BGP Graceful Restart,
it MUST NOT send a BoRR for an <AFI, SAFI> to a neighbor before it sends
the EoR for the <AFI, SAFI> to the neighbor." But it doesn't forbid
route-refresh request to be sent before receiving EoR.

To handle this scenario, postpone response to refresh request until EoR
is sent.

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>