Fix PDG search

[michamos]

Several changes to the query parser:

• Values with colons inside are now handled for all keywords. This was not handled previously due to the ambiguity in key foo bar:baz. Is it a search for foo bar:baz in key or a search for foo in key and baz in bar? To handle this, colons are allowed only in the first space-separated value. As a side-effect, this allowed to remove the special-casing for texkey searches.
• Exact matches are delimited by "...". This was previously translated into a term query (for most fields), which is useless for text fields as the search query needs to match exactly the output of the analyzer. Now a match_phrase is used, which should work correctly.
• Search for creation, update and earliest dates are now handled correctly.
• Makes the searching for texkeys more strict. Previously anything containing a : would also search in the texkey field. Now the texkey search is only emitted if the term looks like a texkey.
• INSPIR-3309
• INSPIR-3310