rework query parser according to solr specs

forward slashes are now quoted correctly, and `&&` and `||` boolean operators also work again (essentially reverting b893000c)

please also note that the regex is now using a "raw" string, which allows simpler quoting, in particular for backslahes
adjust tests querying solr directly to properly quote forward slashes
add integration tests to make sure boolean operators work again (see 783e8841)
add tests to demonstrate forward slashes are correctly quoted, including in groups
compare the actual date & time for `DateTime` instances, not their original string representation

this fixes version-dependent test failures related to missing `+0` timezone specifiers