leveldb: Update to v1.15.0

:Release Notes:
Update leveldb to v1.15.0 .

:Detailed Notes:
- Switched from mmap based writing to simpler stdio based writing. Has a
  minor impact (0.5 microseconds) on microbenchmarks for asynchronous
  writes. Synchronous writes speed up from 30ms to 10ms on linux/ext4.
  Should be much more reliable on diverse platforms.
- Compaction errors now immediately put the database into a read-only
  mode (until it is re-opened). As a downside, a disk going out of space
  and then space being created will require a re-open to recover from,
  whereas previously that would happen automatically. On the plus side,
  many corruption possibilities go away.
- Force the DB to enter an error-state so that all future writes fail
  when a synchronous log write succeeds but the sync fails.
- Repair now regenerates sstables that exhibit problems.

:Testing Performed:
miniBAT
db8 unit tests
db8 shard tests

:QA Notes:

:Issues Addressed:
[GF-56129] Update to latest leveldb version 1.15.0 and change sync
           option from async to sync
[GF-56423] CCC: leveldb=v1.15.0

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <maxim.sditanov@lge.com>
Change-Id: I9a36e635dc740fda8a4d45469c986cb8a12b6d39
Reviewed-on: https://g2g.palm.com/4189
Reviewed-by: DCO Verification
Build: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>
Reviewed-by: Herb Kuta <herb.kuta@lge.com>