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.

Along the way, move a brace to the same line with `Buffer` class name to
make it conform to our clang format.

Part of #110

Author: drewdzzz

src/Buffer/Buffer.hpp

@@ -58,9 +58,8 @@ 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>>
-class Buffer
-{
+template <size_t N = 16 * 1024, class allocator = MempoolInstance<N, 16>>
+class Buffer {
 private:
 	/** =============== Block definition =============== */
 	/** Blocks are organized into linked list. */