Prefix scan: db_bench and bug fixes

Summary: If use_prefix_filters is set and read_range>1, then the random seeks will set a the prefix filter to be the prefix of the key which was randomly selected as the target.  Still need to add statistics (perhaps in a separate diff).

Test Plan: ./db_bench --benchmarks=fillseq,prefixscanrandom --num=10000000 --statistics=1 --use_prefix_blooms=1 --use_prefix_api=1 --bloom_bits=10

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb, haobo

Differential Revision: https://reviews.facebook.net/D12273

db/table_cache.h

@@ -52,6 +52,12 @@ class TableCache {
              void (*mark_key_may_exist)(void*) = nullptr,
              const bool no_io = false);
 
+  // Determine whether the table may contain the specified prefix.  If
+  // the table index of blooms are not in memory, this may cause an I/O
+  bool PrefixMayMatch(const ReadOptions& options, uint64_t file_number,
+                      uint64_t file_size, const Slice& internal_prefix,
+                      bool* table_io);
+
   // Evict any entry for the specified file number
   void Evict(uint64_t file_number);