SkipListRep::LookaheadIterator

Summary:
This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an
optimization for the tailing use case which includes many seeks. E.g. consider
the following operations on a skip list iterator:

   Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ...

If `lookahead` is positive, `SkipListRep` will return an iterator which also
keeps track of the previously visited node. Seek() then first does a linear
search starting from that node (up to `lookahead` steps). As in the tailing
example above, this may require fewer than ~log(n) comparisons as with regular
skip list search.

Test Plan:
Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It
first writes N records (with consecutive keys), then measures how much time it
takes to read them by calling `Seek()` and `Next()`.

   $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \
      -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \
      -seekseq_next 2 -skip_list_lookahead=0
   [...]
   DB path: [/dev/shm/rocksdbtest/dbbench]
   fillseekseq  :       0.389 micros/op 2569047 ops/sec;

   real    0m21.806s
   user    0m12.106s
   sys     0m9.672s

   $ time ./db_bench [...] -skip_list_lookahead=2
   [...]
   DB path: [/dev/shm/rocksdbtest/dbbench]
   fillseekseq  :       0.153 micros/op 6540684 ops/sec;

   real    0m19.469s
   user    0m10.192s
   sys     0m9.252s

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb, march, lovro

Differential Revision: https://reviews.facebook.net/D23997