Count number of write transactions in stats

[NeonWizard]

Stats only seems to keep count of read transactions (Stats.TxN, Stats.OpenTxN). Stats should also keep count of write transactions.

[Elbehery]

@ahrtr shall I take this ?

[ahrtr]

bbolt only supports one RW TXN at a time, so it only makes sense to add a RWTxN to record the total number of RW TXN which have ever started since opening the db.

[Elbehery]

Yes i investigated this and found that it makes no sense to count WTx as it is always one 👍🏽