Add close_file arg to saxdb_close_context(); allocate all saxdb contexts from heap.

Add close_file arg to saxdb_close_context(); allocate all saxdb contexts from heap.

src/saxdb.h (saxdb_close_context): Add "close_file" argument.
src/saxdb.c (saxdb_write_db): Use saxdb_{open,close}_context() rather than
    putting it on the stack.
  (write_database): Likewise.
  (saxdb_close_context): Implement the close_file argument.
src/modcmd.c (cmd_dump_messages): Use the close_file argument.
src/mod-helpserv.c (helpserv_log_request): Likewise.