Use decimal objects for fees in mempool_accept.py

Summary:
Use python Decimal when defining fees and amounts. This ensures that subsequent operations on these amounts produces exact decimal amount, without potential rounding errors, instead of floats.

This is a backport of core#20039

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10599