fix(lib): use int64 for mempool bytes counter to prevent overflow

[yasinlex]

Problem

The mempool's txsBytes counter used int type, which is 32 bits on 32-bit systems. This could overflow when accumulating large transactions, potentially causing:

1. Incorrect mempool size tracking
2. Failure to enforce MaxTotalBytes limit
3. Unexpected behavior in transaction dropping logic

Changes

// Before
type FeeMempool struct {
    pool     MempoolTxs
    txsBytes int           // 32-bit on 32-bit systems
    config   MempoolConfig
}

// After
type FeeMempool struct {
    pool     MempoolTxs
    txsBytes int64         // Always 64-bit
    config   MempoolConfig
}

Updated all internal calculations to use int64:

• AddTransactions(): local txsBytes variable
• DeleteTransaction(): convert deleted bytes to int64
• Drop loop: convert dropped bytes to int64

The TxsBytes() method still returns int for backward compatibility with the Mempool interface.

Impact

• Before: Potential integer overflow on 32-bit systems with >2GB of transactions
• After: Safe handling of large transaction volumes up to 9.2 exabytes