timeout_config: introduce timeout configuration

Different request types have different timeouts (for example,
read requests have shorter timeouts than truncate requests), and
also different request sources have different timeouts (for example,
an internal local query wants infinite timeout while a user query
has a user-defined timeout).

To allow for this, define two types: timeout_config represents the
timeout configuration for a source (e.g. user), while
timeout_config_selector represents the request type, and is used
to select a timeout within a timeout configuration. The latter is
implemented as a pointer-to-member.

Also introduce an infinite timeout configuration for internal
queries.

database.cc

@@ -4605,3 +4605,8 @@ std::ostream& operator<<(std::ostream& os, gc_clock::time_point tp) {
     tmp << std::setw(12) << sec;
     return os;
 }
+
+const timeout_config infinite_timeout_config = {
+        // not really infinite, but long enough
+        1h, 1h, 1h, 1h, 1h, 1h, 1h,
+};