Use piecewise Iteration for a full table scan.

[prataprc]

Full table scan on active tree will have following issues:

• With MVCC disabled, Iterations will lock the entire tree, until it
  completes.
• With MVCC enabled, Iterations won't lock the writer and won't
  interfer with other concurrent readers. But if there are hundreds
  and thousands of mutations happening in the background, while the
  iterator is holding the snapshot, it can lead to huge memory
  pressure.

To avoid this, implement a PiecewiseIterator that scans the tree
part by part and stitches them together to simulate a full-table scan.
Here are some implementation guidelines.

• Can be implemented only when application can maintain a monotonically
  increasing seqno for every mutation (CREATE, UPDATE, DELETE).
• LLRB should be created with metadata.bornseqno, metadata.deadseqno
  enabled.
• PiecewiseIterators will repeat a large number of small Iteration on
  the tree until it completes a full table scan.
• Application should supply the tillseqno as a form of timestamp to
  filter mutations that happens after the point in time that started
  PiecewiseIteration.
• The first iteration will have startkey as nil and endkey as nil.
• Each iteration will read only 1000 entries. And the last key will be
  remembered as the startkey for the next iteration with inclusion
  set to high.
• For every iteration, the entries read from the tree will be filtered
  and returned to the application.
  • Get the node's timestamp by picking the larger value between
    bornseqno and deadseqno.
  • Node's timestamp should be less than or equal to tillseqno. Else
    skip.

The key difference between Iterator and PiecewiseIterator is that
PiecewiseIterator does not give a point time view of the tree snapshot.
For example, if an entry that is not yet read by the piecewise-iterator
is updated after the iteration has started, then the entry might be skipped
and this entry won't be part of the final output.

This implies:

• The final output won't contain the full sample set of entries.
• Cannot be queried with correctness.

Hopefully it can still be used with LSM reads.