Extend the set of flags of the aggregator to refine the application pools management

[AntoineRondelet]

In former versions of the project, parameters such as the depth of the application pool were defined in the zecale_constants.hpp file (https://github.com/clearmatics/zecale/blob/v0.3/libzecale/zecale_constants.hpp#L15). However, such values do not represent protocol constants, but rather configuration values that may be adjusted from one run to another.
As such, such values need to be managed and passed at the CLI level via flags (that may overwrite values in an underlying configuration flag).
Useful flags may be:

--app-pool-depth <define the max depth of the application pools>
--app-pool-ordering-policy <enumerate values that may be used to order nested txs in the pool>

but also, things like --base-fee to define the minimum fees to make it to the pool (related to: #49) etc.