Fix a number of object lifetime/ownership issues

Summary:
Replace manual memory management with std::unique_ptr in a
number of places; not exhaustive, but this fixes a few leaks with file
handles as well as clarifies semantics of the ownership of file handles
with log classes.

Test Plan: db_stress, make check

Reviewers: dhruba

Reviewed By: dhruba

CC: zshao, leveldb, heyongqiang

Differential Revision: https://reviews.facebook.net/D8043

db/log_reader.cc

@@ -15,9 +15,9 @@ namespace log {
 Reader::Reporter::~Reporter() {
 }
 
-Reader::Reader(SequentialFile* file, Reporter* reporter, bool checksum,
-               uint64_t initial_offset)
-    : file_(file),
+Reader::Reader(unique_ptr<SequentialFile>&& file, Reporter* reporter,
+               bool checksum, uint64_t initial_offset)
+    : file_(std::move(file)),
       reporter_(reporter),
       checksum_(checksum),
       backing_store_(new char[kBlockSize]),