Enforce limits for storage operations

[kostko]

Currently the client can submit arbitrarily large batches of storage operations which can cause significant load on the storage node. We should review all operations and make sure that they have sensible limits in place.

• Limit the depth of forked roots to prevent increasing the number of passes that need to be done in Finalize. The depth should currently be limited to 2 as this is the maximum depth that is currently generated during normal operation.
• Limit the size of storage batches in Apply.
• Limit the amount of Apply operations in a round based on identity and role of caller (e.g., a txn scheduler node may only commit up to the number of compute committees I/O roots, a compute node may only commit 1 I/O root and 1 state root, a merge node may only merge).

Estimated cost: 1 sprint