grammar.y: Added some (very) short documentation for each statement.
store: Added sdb_store_iterate().

This function iterates over all (host, for now) objects in the store and calls
a user-specified callback for each.
store: Added sdb_store_lookup().

This function looks up objects from the store which match a user-specified
matcher. For each such object, a user-specified callback is executed.
store: Let sdb_store_matcher_matches return true if it matches.

Rather than using 0 for matches and -1 else.
frontend: Added simple 'LOOKUP <type> WHERE <expression>' query.

This query command may be used to retrieve a list of detailed host information
for each host matching the specified expression. For now, querying hosts is
supported only.
frontend: Added LOOKUP support to the connection handler.

For now, the wire format is: <CONNECTION_LOOKUP>;<len>;<expression>
parser_test: Added simple LOOKUP parser tests.