Make the TTL for disk cache configurable, we can now completely disable

disk cache lookup for example by doing:
  nix copy --from <binary-cahe> <store-path> --option \
  positive-disk-cache-ttl 0

Issues: #1885 #2035

src/libstore/nar-info-disk-cache.cc

@@ -1,6 +1,7 @@
 #include "nar-info-disk-cache.hh"
 #include "sync.hh"
 #include "sqlite.hh"
+#include "globals.hh"
 
 #include <sqlite3.h>
 
@@ -47,10 +48,6 @@ class NarInfoDiskCacheImpl : public NarInfoDiskCache
 {
 public:
 
-    /* How long negative and positive lookups are valid. */
-    const int ttlNegative = 3600;
-    const int ttlPositive = 30 * 24 * 3600;
-
     /* How often to purge expired entries from the cache. */
     const int purgeInterval = 24 * 3600;
 
@@ -116,8 +113,8 @@ public:
                 SQLiteStmt(state->db,
                     "delete from NARs where ((present = 0 and timestamp < ?) or (present = 1 and timestamp < ?))")
                     .use()
-                    (now - ttlNegative)
-                    (now - ttlPositive)
+                    (now - settings.ttlNegativeDiskCache)
+                    (now - settings.ttlPositiveDiskCache)
                     .exec();
 
                 debug("deleted %d entries from the NAR info disk cache", sqlite3_changes(state->db));
@@ -186,8 +183,8 @@ public:
             auto queryNAR(state->queryNAR.use()
                 (cache.id)
                 (hashPart)
-                (now - ttlNegative)
-                (now - ttlPositive));
+                (now - settings.ttlNegativeDiskCache)
+                (now - settings.ttlPositiveDiskCache));
 
             if (!queryNAR.next())
                 return {oUnknown, 0};