Mempool: bound number and total size of valid operations of all kinds

Currently, the plugin ensures that the number of valid manager operations in the mempool is at most max_prechecked_manager_operations = 5000, with no constraints on the other kinds of operations.

Instead, we want to ensure that the number of valid operations of any kind is at most some configurable max_count, and also the sum of the sizes in bytes of all valid operations is at most some configurable max_total_size.

Moreover, there is no good reason for the mempool bounding to be handled in the plugin. It should be done in lib_shell to reduce the reliance on the plugin.