Web indexing: add a parameter to specify at what frequency a page should be stored again: none/day/week/month/year. Previously, only one instance could be kept, which is still the default

src/sampleconf/recoll.conf

@@ -545,6 +545,16 @@ webcachemaxmbs = 40
 # then moved by a script to webqueuedir.</descr></var>
 #webdownloadsdir = ~/Downloads
 
+# <var name="webcachekeepinterval" type="string">
+#
+# <brief>Page recycle interval</brief>
+# <descr>By default, only one instance of an URL is kept in the cache. This
+# can be changed by setting this to a value determining at what frequency
+# we keep multiple instances ('day', 'week', 'month',
+# 'year'). Note that increasing the interval will not erase existing
+# entries.</descr></var>
+#webcachekeepinterval=
+
 # <var name="aspellDicDir" type="dfn">
 #
 # <brief>Aspell dictionary storage directory location.</brief> <descr>The