Our net socket connection allocations are powerful but redundant
because they're implemented once in each protocol. This is not good for
further optimizing and extending to other allocations, so maybe we can
add a common implementation for the usage.
Impact:
1. We add a `struct net_bufpool_s` as pool descriptor, which may use a
little bit more memory than previous implementation (~28Bytes).
2. We share same functions between pools, so code size may shrink under
some scenarios.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
