sql: refactor memory allocation system

This patch refactors the SQL memory allocation system. There are three
main changes:
1) now, when allocating memory, no additional 8 bytes are allocated to
remember the size of the allocated memory, so instead of
sql_malloc()/sqlRealloc()/sql_free(), the malloc()/realloc()/free()
functions are used;
2) the malloc()/realloc() functions were used through the
xmalloc()/xrealloc() macros, so checks for memory allocation errors were
removed;
3) there is no need for an explicit "sql *db" argument for most of the
functions, so it has been omitted.

Part of #1544

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring