Support EXPLAIN FORMAT=JSON like MySQL 5.6 does.

Difference from MySQL's EXPLAIN FORMAT=JSON

We don't want to copy MySQL 5.6:
From the user point of view:

• 5.6 output format is not documented and unstable (even MySQL Workbench fails to parse it in some cases)
  • I don't expect that any 3rd party is able to parse it, other than relying that it's a well-formed JSON document.
• 5.6 output format doesn't allow to display some info we want to display
• 5.6's output format has legacy of tabular output format in many places
• MariaDB has more optimizer features, so we will have to produce output that 5.6 never produces

Currently supported

• Basic SELECTs
• Table access methods
  • full table/index scans
  • range access
  • index_merge access
  • I_S read optimizations
• "Using where" (attached_condition)
• rows and 'filtered' columns
• Index Condition Pushdown ('index_condition')
• UNIONs (incomplete)
• Item-based subqueries (incomplete)
• Basic join buffering
• Single-table UPDATE/DELETE
• Derived tables
• Non-merged semi-joins (JTBMs)
• [Merged] Semi-joins: SJ-Materialization
• [Merged] Semi-joins: FirstMatch, DuplicateElimination, LooseScan

Not yet supported (important)

• ORDER BY/GROUP BY/ DISTINCT handling – as agreed won't be ready before 10.1.2

Not yet supported (less important)

• Subquery cache feature
• Advanced join buffering (display MRR as a special kind of scan)