Use a 64bit counter for the records written during the backup

[hvlad]

Two quick questions:

• did you test if really large values (>2^32) printed correctly ?
• what about restore ?

[dmitry-starodubov]

I did not test it on a real database, but changed the value of records variable in the debugger to 5*10^9. The output was correct. 64bit counters are already used in restore code.

[dyemanov]

Frontported to v6.