…:Reserve

ColumnLowCardinality assumes that not all items are unique,
  hence dictionary column can be reserved for smaller capacity;
ColumnString now allows to set average value size estimation
  in constructor or on existing instance.
  If estimation is close to real average value size, then memory is pre-allocations are close to optimum.

…tests

- Added validation for ColumnString::EstimatedValueSize value in ColumnString
- Minor refactoring of block allocation in ColumnString
- Moved test code for ColumnString and ColumnLowCardinality to separate files,
- Added more test cases

Andded PrettyPrintByteSize + some tests

also removed commented out code

…oncrete columns