Make Cache::GetCapacity constant

Summary: This will allow us to access constant via `DB::GetOptions().table_cache.GetCapacity()` or `DB::GetOptions().block_cache.GetCapacity()` since GetOptions() is also constant method.

include/rocksdb/cache.h

@@ -102,7 +102,7 @@ class Cache {
   virtual uint64_t NewId() = 0;
 
   // returns the maximum configured capacity of the cache
-  virtual size_t GetCapacity() = 0;
+  virtual size_t GetCapacity() const = 0;
 
  private:
   void LRU_Remove(Handle* e);

util/cache.cc

@@ -404,7 +404,7 @@ class ShardedLRUCache : public Cache {
     MutexLock l(&id_mutex_);
     return ++(last_id_);
   }
-  virtual size_t GetCapacity() {
+  virtual size_t GetCapacity() const {
     return capacity_;
   }
 };