Display number of flushed entries

[pepyakin]

We want to enable the constraint authors to see the costs of a particular constraint system / table. A big factor influencing the cost of proving is flushes. Flush is some sort of rule that is attached to a table. That rule requires putting a subset of columns into the channel.

The number of flushes per table multiplied by the table capacity should estimate the prover cost sufficiently.

Now some details that might be helpful:

1. The flushes are stored per TablePartition.
2. The number of flushes per row might be greater than one due to multiplicity.
3. The way we report the costs is via the TableStat struct.

Feel free to ask questions here or reach out @pepyakin on telegram.