omnipool initialize limit

with recent introduction of slip fee based, we need to clear up a storage in on_finalize

for this purpose, we need to allocate a weight for this operation in on_initialize

Currently, there is iteration of all assets in omnipool - and it is not bounded.

with current state of omnipool - this is no issue but in theory, although very unrealistic, it could take a lot of block weight.

Introduce a max limit or propose another solution.