Speed up route_check script

[deepak-singhal0408]

What I did

This PR fixes sonic-net/sonic-buildimage#18773

How I did it

Parallely execute route_check on each Asic.
Parallelly fetch ipv4 routes and ipv6 routes.

How to verify it

execute "time route_check.py" on T2 chassis having 32k v4+32k v6 routes.
Results:
Before:
Checking routes for namespaces: ['asic0', 'asic1']

real 3m16.387s
user 1m26.084s
sys 0m7.275s

After:
time route_check.py
real 1m30.675s
user 1m33.777s
sys 0m8.209s

Previous command output (if the output of a command-line utility has changed)

None

New command output (if the output of a command-line utility has changed)

None

[deepak-singhal0408]

@anamehra , could you try this change and see if this helps? Thanks!