ISSU: Enhancements to minimize data plane downtime

Vrouter Kernel Module allocates large amount of memory for Flow Tables and Bridge Tables. Being Hash Tables, the memory for these need to be contiguous to hash into the buckets. Vrouter currently uses B-tables to divide the allocation of memory into multiple chunks of 4MB. In spite of this, the allocation can still fail in a running system, creating the constraint of allocating the memory immediately after boot up. This decreases the possibility of memory allocation failure but loses the flexibility of inserting the Vrouter module whenever desired. This feature is to remove this constraint.