bgpd: bgp_sync_label_manager failure case

[donaldsharp]

There are several problems with the bgp_sync_label_manager function:

a) It is possible that a request in the lp->requests fifo will be unable to be filled at this point in time and the lf will be leaked and not ever fullfilled.

b) The bgp_sync_label_manager runs one time a second irrelevant if there is work to do or not.

To fix (a) just add the request back to the requests fifo and set the timer to pop in the future.

To fix (b) just every time something is put into
the request pool start a timer to run in 1 second
and do not restart it if all the work is done.

[donaldsharp]

As a note I chased this down because I was looking at a show thread cpu and saw this line:

    1     104315.384   2682787       38       608       42      4795         0         0         11    T    bgp_sync_label_manager

bgp_sync_label_manager has been run 2.6 million times on my box, which is effectively 31 days of FRR uptime. This event alone has consumed 104 seconds of cpu! I do not even have any label configuration on this box. So this is extremely wasteful.