ip: add support to preallocate pools

ip: add support to preallocate pools

Type: improvement

In certain use cases, the underlying pools expand by allocating a new,
larger pool and copying the existing elements into it. This process
can be time-consuming, leading to slower control plane configurations,
especially when a large number of elements are already present.
This patch allows users to pre-configure some of these pools through
startup.conf.
It also fixes alignment for ip4 mtrie.

Signed-off-by: Mohsin Kazmi <[email protected]>
Signed-off-by: Benoît Ganne <[email protected]>
Change-Id: Ib0f1d40e3efb8b4fce989219196c718d6834498a