SequenceBuilder: add .Reset() method and use it during merge

[AskAlexSharov]

Problem:

In PR:

• re-use 1 Builder object for many EF merges and many EF builds (in II.mergeFiles)
• moved Merge method from Reader to Builder
• moved reusable iterators for Merge method inside Builder object
• “alloc=2.7g sys=5.4g” -> “alloc=2.7g sys=2.7g” (during large .rf file merge)