Fix deadlock in WAL sync

Fix deadlock in WAL sync

Summary:
MarkLogsSynced() was doing `logs_.erase(it++);`. The standard is saying:

```
all iterators and references are invalidated, unless the erased members are at an end (front or back) of the deque (in which case only iterators and references to the erased members are invalidated)
```

Because `it` is an iterator to the first element of the container, it is
invalidated, only one iteration is executed and `log.getting_synced = false;`
is not being done, so `while (logs_.front().getting_synced)` in `WriteImpl()`
is not terminating.

Test Plan: make db_bench && ./db_bench --benchmarks=fillsync

Reviewers: igor, rven, IslamAbdelRahman, anthony, kradhakrishnan, yhchiang, sdong, tnovak

Reviewed By: tnovak

Subscribers: kolmike, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D45807