Cache: Introduce soft and hard TTL

- Hard TTL; the maximum time a value will be used from the cache.
- Soft TLL; the time after which the cache value should be updated
  from the server.

So as not to unnecessarily fetch the same value multiple times, when
the soft TTL has been reached, the value will be updated for the first
client, while all other clients will use the stale value until it has
become updated.

With different soft and hard TTLs there is a definite upper bound for
how old a value can be used.

Documentation/Filters/Cache.md

@@ -13,7 +13,8 @@ existing service.
 [Cache]
 type=filter
 module=cache
-ttl=5
+hard_ttl=30
+soft_ttl=20
 storage=...
 storage_options=...
 rules=...
@@ -59,6 +60,34 @@ depend upon the specific module. For instance,
 storage_options=storage_specific_option1=value1,storage_specific_option2=value2
 ```
 
+#### `hard_ttl`
+
+_Hard time to live_; the maximum amount of time - in seconds - the cached
+result is used before it is discarded and the result is fetched from the
+backend (and cached). See also `soft_ttl` below.
+
+```
+hard_ttl=60
+```
+The default value is `0`, which means no limit.
+
+#### `soft_ttl`
+
+_Soft time to live_; the amount of time - in seconds - the cached result is
+used before it is refreshed from the server. When `soft_ttl` has passed, the
+result will be refreshed when the _first_ client requests the value.
+
+However, as long as `hard_ttl` has not passed, _all_ other clients requesting
+the same value will use the result from the cache while it is being fetched
+from the backend. That is, as long as `soft_ttl` but not `hard_ttl` has passed,
+even if several clients request the same value at the same time, there will be
+just one request to the backend.
+```
+soft_ttl=60
+```
+The default value is `0`, which means no limit. If the value of `soft_ttl` is
+larger than `hard_ttl` it will be adjusted down to the same value.
+
 #### `max_resultset_rows`
 
 Specifies the maximum number of rows a resultset can have in order to be
@@ -78,16 +107,6 @@ max_resultset_size=128
 ```
 The default value is `0`, which means no limit.
 
-#### `ttl`
-
-_Time to live_; the amount of time - in seconds - the cached result is used
-before it is refreshed from the server.
-
-```
-ttl=60
-```
-The default value is `0`, which means no limit.
-
 #### `max_count`
 
 The maximum number of items the cache may contain. If the limit has been