sql: display decoded msgpack for EXPLAIN queries

During DDL routines we pass encoded space/index/trigger formats
into msgpack to VDBE. EXPLAIN query displays arguments of each opcode of
VDBE program in a readable format. So, lets decode arguments of OP_Blob
opcode with subtype = _MSGPACK before displaying them. Also, lets
enlarge static buffers for P4 operand value and opcode comment to fit
decoded msgpack.

What is more, it fixes buffer-overflow since before this patch operands
of OP_Blob were treated as strings and passed to functions like strlen()
(only during EXPLAIN query). On the other hand, generally speaking
msgpack can come without null termination, or contain '\0' symbols in
the middle of encoded array.

Closes #3868