Remove MemTableRep::GetIterator(const Slice& slice)

Summary: It seems to me that when ever function MemTableRep::GetIterator(const Slice& slice) is used, we can use MemTableRep::GetDynamicPrefixIterator() instead. Just delete it to simplify the codes.

Test Plan: make all check

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: xjin, dhruba, haobo, leveldb

Differential Revision: https://reviews.facebook.net/D19281
Fixed a comparison between signed and unsigned integers in options.cc

Summary:
Fixed the following warning:

util/options.cc: In constructor ‘rocksdb::ColumnFamilyOptions::ColumnFamilyOptions(const rocksdb::Options&)’:
util/options.cc:157:58: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
   if (max_bytes_for_level_multiplier_additional.size() < num_levels) {
                                                             ^

Test Plan: make all check

Reviewers: haobo, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19293
Cache some conditions for DBImpl::MakeRoomForWrite

Summary:
Task 4580155. Some conditions in DBImpl::MakeRoomForWrite can be cached in
ColumnFamilyData, because theirs value can be changed only during compaction,
adding new memtable and/or add recalculation of compaction score.

These conditions are:

cfd->imm()->size() ==  cfd->options()->max_write_buffer_number - 1
cfd->current()->NumLevelFiles(0) >=  cfd->options()->level0_stop_writes_trigger
cfd->options()->soft_rate_limit > 0.0 &&
    (score = cfd->current()->MaxCompactionScore()) >  cfd->options()->soft_rate_limit
cfd->options()->hard_rate_limit > 1.0 &&
    (score = cfd->current()->MaxCompactionScore()) >  cfd->options()->hard_rate_limit

P.S.
As it's my first diff, Siying suggested to add everybody as a reviewers
for this diff. Sorry, if I forgot someone or add someone by mistake.

Test Plan: make all check

Reviewers: haobo, xjin, dhruba, yhchiang, zagfox, ljin, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19311