[[s3 perf]]: make precharge work with ext-blobs

[SammyVimes]

External blobs are not being precharged if read with read iterator.
Without precharging each external blob is requested separately.
Measured speed up is 2 times.

[SammyVimes]

#5758

[snaury]

Thankfully this hasn't been merged into stable branches yet, there may be a critical bug: #7769.