Fixed cache key for block cache

Summary:
Added function to `RandomAccessFile` to generate an unique ID for that file. Currently only `PosixRandomAccessFile` has this behaviour implemented and only on Linux.

Changed how key is generated in `Table::BlockReader`.

Added tests to check whether the unique ID is stable, unique and not a prefix of another unique ID. Added tests to see that `Table` uses the cache more efficiently.

Test Plan: make check

Reviewers: chip, vamsi, dhruba

Reviewed By: chip

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8145

util/coding.h

@@ -18,6 +18,10 @@
 
 namespace leveldb {
 
+// The maximum length of a varint in bytes for 32 and 64 bits respectively.
+const unsigned int kMaxVarint32Length = 5;
+const unsigned int kMaxVarint64Length = 10;
+
 // Standard Put... routines append to a string
 extern void PutFixed32(std::string* dst, uint32_t value);
 extern void PutFixed64(std::string* dst, uint64_t value);