Mempool: adjust sender constraints

[andreibancioiu]

Reasoning behind the pull request

• When adding transactions in the mempool, the addition is subject to the so-called "sender constraints". Historically, the parameters that guide the constraints were these.
• However, since, for some time, the interceptors apply this constraint, it's reasonable to adjust the "sender constraints", as well.
• Mempool: add benchmarks for addition in worst case mx-chain-storage-go#64

Proposed changes

• Adjust TxDataPool.SizePerSender to a lower value.

Testing procedure

• Standard testing.

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?