Problem:
We currently have no way of limiting memory consumption when parsing
SQL queries. With a large value for max_allowed_packet it is easy to
construct queries which require several gigabytes of memory during
parsing.

Solution:
We build on the MEM_ROOT extensions developed for bug
Bug#17769777 : EXCESSIVE MEMORY CONSUMPTION WITH MANY AND / BETWEEN ITEMS

Introduce a maximum capacity for the MEM_ROOT used by the parser,
and raise an error if this capacity is exceeded during parsing.
If the lexer sees a ER_CAPACITY_EXCEEDED error, it returns ABORT_SYM
and parsing of the current query is terminated.

The memory capacity can be set with the new system variable
'parser_max_mem_size'. By default there is no limit, i.e. the variable is
set to 4294967295 bytes on 32bit systems, 18446744073709551615ULL
on 64bit systems. The database administrator may set
@@global.parser_max_mem_size to a lower value, in order to protect
against out-of-memory situations caused by parsing long/complex SQL queries.