Remove catastrophic backtracking in regex

This updates the regular expression so that there is no longer a chance
for it to end up catastrophically backtracking and locking up the
process.

waitress/rfc7230.py

@@ -40,13 +40,13 @@
 #                  field-vchar ]
 
 FIELD_VCHAR = "[" + VCHAR + OBS_TEXT + "]"
-FIELD_CONTENT = FIELD_VCHAR + "([ \t" + VCHAR + OBS_TEXT + "]+" + FIELD_VCHAR + "){,1}"
-FIELD_VALUE = "(" + FIELD_CONTENT + "){0,}"
+# Field content is more greedy than the ABNF, in that it will match the whole value
+FIELD_CONTENT = FIELD_VCHAR + "+(?:[ \t]+" + FIELD_VCHAR + "+)*"
+# Which allows the field value here to just see if there is even a value in the first place
+FIELD_VALUE = "(?:" + FIELD_CONTENT + ")?"
 
 HEADER_FIELD = re.compile(
     tobytes(
         "^(?P<name>" + TOKEN + "):" + OWS + "(?P<value>" + FIELD_VALUE + ")" + OWS + "$"
     )
 )
-
-OWS_STRIP = re.compile(OWS + "(?P<value>.*?)" + OWS)