[ZKS-04] Fixes the maximum transmissions per batch, increases the transactions per block

[howardwu]

Motivation

• Reduces the maximum transmissions per batch.
  • This limit is set to 50 as part of safety measures to prevent DoS attacks.
  • This limit can be increased in the future as performance improves. Alternatively, the rate of block production can be sped up to compensate for the limit set here.
• Increases the number of transactions per block to 2^20 to reflect subdag safety (overflow) requirements

The sister PR in snarkOS can be found here: https://github.com/AleoHQ/snarkOS/pull/2869.

Audit Finding: [zksecurity 04] Garbage Collection Can Block Commits From Happening