lib: Fix to optimize the time taken while batching huge configs

lib: Fix to optimize the time taken while batching huge configs

Issue: When the incoming config has say 30K entries of a prefix-lists,
current implementation is to schedule the configs to be batched and
only after batching the entire config, the processing of the configs
take place. As part of batching this config, we perform string
concatenation to save all the configs in the buffer which over time
results in taking longer time.

Ex: Imagine each line of config is 50 chars. With a delimiter of ‘- ‘ we end
up adding 52 chars to buffer for each command i.e. 52*30000 = 156K of chars.
Strlcat is an expensive operation and every time we strlcat, we have to
traverse at end of string to append new char.
Because of this, we end up adding extra 6-8 secs for accepting the config.

Fix: The idea here is to bring back something similar to the backoff
count implemented as part of 20e9a402 (lib: introduce configuration
back-off timer for YANG-modeled commands).

Essentially we keep a cap of 5000 per batch. So once 5000k config
commands are batched, we process them, clear the buffer, set the count
to 0 and then continue processing the rest of the config.

option1 file has 30K entries of prefix-list
Without Fix:
root@mlx-3700-20:mgmt:/var/log/raja/frr# time sudo vtysh -f option1
<SNIP>..............
Waiting for children to finish applying config...
[25191|staticd] done
[25189|watchfrr] done
[25178|ospfd] done
[25190|pbrd] done
[25181|bgpd] done
[25175|zebra] done

real    0m20.123s
user    0m9.384s
sys     0m2.403s

With Fix:
root@mlx-3700-20:mgmt:/var/log/raja/frr# time sudo vtysh -f option1
<SNIP>..............
Waiting for children to finish applying config...
[19887|staticd] done
[19885|watchfrr] done
[19886|pbrd] done
[19874|ospfd] done
[19877|bgpd] done
[19871|zebra] done

real	0m12.168s
user	0m7.511s
sys	0m1.981s

Issue: 3589101

Ticket# 3589101

Signed-off-by: Rajasekar Raja <rajasekarr@nvidia.com>