frontend parser: Added support for AND and OR matchers.
parser_test: Check matcher type returned by sdb_fe_parse_matcher().
parser_test: Check operator precedence.
store: Don't change the interval if the new interval is zero.

If the interval is zero, multiple updates happened at the same time. This does
not affect the interval at which the object is updated in the backend, though.
frontend: Fixed memory holes in the parser.
frontend: Let the parser support brackets in matcher expressions.
store_lookup: Added sdb_store_inv_matcher().

This creates a matcher objects which matches the inverse (logical NOT) of the
specified matcher.
core, frontend: Added !=, !~, and NOT operators.
frontend: Actually implement support for the NOT operator.