fatxpool: size limits implemented

[michalkucharczyk]

This PR adds size-limits to the fork-aware transaction pool.

Review Notes

• Existing TrackedMap is used in internal mempool to track the size of extrinsics:
  

  polkadot-sdk/substrate/client/transaction-pool/src/graph/tracked_map.rs

  Lines 33 to 41 in 58fd5ae

  	/// Map with size tracking.
  	///
  	/// Size reported might be slightly off and only approximately true.
  	#[derive(Debug)]
  	pub struct TrackedMap<K, V> {
  	index: Arc<RwLock<HashMap<K, V>>>,
  	bytes: AtomicIsize,
  	length: AtomicIsize,
  	}

• In this PR, I also removed the logic that kept transactions in the tx_mem_pool if they were immediately dropped by the views. Initially, I implemented this as an improvement: if there was available space in the mempool and all views dropped the transaction upon submission, the transaction would still be retained in the mempool.

  However, upon further consideration, I decided to remove this functionality to reduce unnecessary complexity. Now, when all views drop a transaction during submission, it is automatically rejected, with the submit/submit_and_watch call returning ImmediatelyDropped.

Closes: #5476

[skunert]

I like the removal of the initial_views, I think the scenario was relatively niche 👍