IP Allocator refactor

[elitegreg]

Summary of Changes

• Changes ip_allocator so that instead of allocating fixed length NetworkV4s, it allocates the number of addresses passed. e.g. allocate(buffer, 1) for /32 and allocate(buffer, 2) for /31
• Updated call sites to pass the number to allocate.
• Behavior of iter_allocated on the allocator changed to return each individual /32. Before if /31s were allocated, a single tunnel allocation (2 addresses) would only list once in iter_allocated. Due to the implementation change, this now must list each /32.
• Updated tests

Testing Verification

• unit tests pass