More ipallocator refactoring

Now x1.5 faster

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>
Implement allocating IPs from CIDR within bridge network

Fixes #4986

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>
Adding additional documentation for fixed-cidr networking

Docker-DCO-1.1-Signed-off-by: Frederick F. Kautz IV <fkautz@alumni.cmu.edu> (github: fkautz)

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>
Merge pull request #6101 from LK4D4/ip_range_#4986

Implement allocating IPs from CIDR within bridge network