ForwardIterator seek bugfix

Summary:
If `NeedToSeekImmutable()` returns false, `SeekInternal()` won't reset the
contents of `immutable_min_heap_`. However, since it calls `UpdateCurrent()`
unconditionally, if `current_` is one of immutable iterators (previously popped
from `immutable_min_heap_`), `UpdateCurrent()` will overwrite it. As a result,
if old `current_` in fact pointed to the smallest entry, forward iterator will
skip some records.

Fix implemented in this diff pushes `current_` back to `immutable_min_heap_`
before calling `UpdateCurrent()`.

Test Plan:
New unit test (courtesy of @lovro):
   $ ROCKSDB_TESTS=TailingIteratorSeekToSame ./db_test

Reviewers: igor, dhruba, haobo, ljin

Reviewed By: ljin

Subscribers: lovro, leveldb

Differential Revision: https://reviews.facebook.net/D19653