Joe: show buffers number in bytes + show all the numbers that are present in the plan

Goal

3 goals for the end-user:

• always be able to see all the numbers related to the operations with shared buffers (blocks), in the order: hit, ready, dirtied, written, even if some of them are 0;
• be able to see "local buffers hit/read/dirtied/written" and "tmp buffers read/written" (see the full list at https://gitlab.com/postgres/postgres/blob/master/src/backend/commands/explain.c#L2937) -- if those numbers are 0, no need to show them
• additionally, understand how much equivalent in bytes it is (for example, 1024 buffer hits is equivalent to 1024 * 8 KiB = 8 MiB, however, some buffers might be "hit" multiple times, it would be good to indicate that).

In the future we will extend it with:

• IO timing (I/O Read Time and I/O Write Time, Postgres can provide it in EXPLAIN ANALYZE);
• real disk IO buffers numbers and timing -- this we will need to discuss separately.

TODO / How to implement

New form to represent the data in the footer of Joe's response:

----
Planning time: 0.719 ms
Execution time: 3.969 ms
Total Cost: 8.45
Buffers
  - shared hit:     100   -- ~800 kiB; from the buffer pool;
                          -- some buffers might be hit multiple times
  - shared read:    23000 -- ~175.48 MiB; from the OS cache, includes disk IO
  - shared dirtied: 2     -- ~16 kiB
  - shared written: 3     -- ~24 kiB
  - temp read:      20    -- ~160 kiB

Acceptance criteria

As a user, I can quickly understand, how much data was read from the buffer pool, how much from the OS cache / disk, how much was dirtied and written, and what were "local" and "temp" buffers amounts, if any.