Fix #31: Entries can disappear when deleted key is reassigned during a migration

Fix #31: Entries can disappear when deleted key is reassigned during a migration

Bug was in the Mutator classes. In the Mutator constructor: If an existing
cell was found, and that cell's value was Redirect, Mutator::m_value was
assigned to Redirect. The constructor would then loop a second time.
On the second iteration of the loop, if no existing cell was found, a new
cell was inserted, but Mutator::m_value was left set to Redirect. This
left the Mutator in an inconsistent state, so that on the next exchangeValue(),
it thought there was a racing write to the same key (which there wasn't), and
neglected to write the new value.

Fix is to avoid ever setting Mutator::m_value to Redirect.