Exec new blocks in TxnNoSync. Prune after sending notifications in TxnSync

[AskAlexSharov]

Change from:

begin(TxNoSync)
exec new block, index nee data
do limited pruning 
commit()
send notifications about new data arrival to other apps, they may start reading new data at this time (by new read transactions) 

Change to:

begin(TxNoSync)
exec new block, index nee data
commit() // no fsync here
send notifications about new data arrival to other apps, they may start reading new data at this time (by new read transactions) 
begin()
do pruning 
commit() // fsync here

it allows notify earlier. Fsync (of all changes) on modern drives is fast, but on cloud-drives it’s about 1sec in worst cases.