Implement $startFrom in stream metadata

$startFrom enable to reastart a Stream that contains bad data.

events with numbers before $startFrom will disapear from the Stream and be scavenged.

it work similary to $maxCount but it is absolute, instead of being relative to the end of the Stream.

It has been implemented in Core StreamMetadata, ReadIndex, TFChunckScavenger, as well as in client API StreamMetadata and its builder.

Unit tests inspired by those for $maxCount have been added.