buffer: use MempoolInstance as default allocator

Currently we use MempoolHolder as default allocator - it makes all
buffers use the same MempoolInstance. This buffer cannot be used in
Multi-Threaded scenarios, so let's simply use `MempoolInstance`
instead - now each connection will use its own allocator. To reduce
memory footprint of each connection, let's set default parameter
`M = slab_size / block_size` to 16 to use 256KB slabs by default.

Part of #110

Author: drewdzzz

src/Buffer/Buffer.hpp

@@ -58,7 +58,7 @@ namespace tnt {
  * REAL_SIZE - constant determines real size of allocated chunk (excluding
  * overhead taken by allocator).
  */
-template <size_t N = 16 * 1024, class allocator = MempoolHolder<N>>
+template <size_t N = 16 * 1024, class allocator = MempoolInstance<N, 16>>
 class Buffer
 {
 private: