Add a 'consensus cache' type on top of storagedir.

Every file in the cache is labeled.  The labels are held in memory;
the bodies are mapped on demand.

src/or/conscache.h

@@ -0,0 +1,48 @@
+/* Copyright (c) 2017, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#ifndef TOR_CONSCACHE_H
+#define TOR_CONSCACHE_H
+
+typedef struct consensus_cache_entry_t consensus_cache_entry_t;
+typedef struct consensus_cache_t consensus_cache_t;
+
+consensus_cache_t *consensus_cache_open(const char *subdir, int max_entries);
+void consensus_cache_free(consensus_cache_t *cache);
+void consensus_cache_unmap_lazy(consensus_cache_t *cache, time_t cutoff);
+void consensus_cache_delete_pending(consensus_cache_t *cache);
+consensus_cache_entry_t *consensus_cache_add(consensus_cache_t *cache,
+                                             const config_line_t *labels,
+                                             const uint8_t *data,
+                                             size_t datalen);
+
+consensus_cache_entry_t *consensus_cache_find_first(
+                                             consensus_cache_t *cache,
+                                             const char *key,
+                                             const char *value);
+
+void consensus_cache_find_all(smartlist_t *out,
+                              consensus_cache_t *cache,
+                              const char *key,
+                              const char *value);
+void consensus_cache_filter_list(smartlist_t *lst,
+                                 const char *key,
+                                 const char *value);
+
+const char *consensus_cache_entry_get_value(const consensus_cache_entry_t *ent,
+                                            const char *key);
+const config_line_t *consensus_cache_entry_get_labels(
+                                          const consensus_cache_entry_t *ent);
+
+void consensus_cache_entry_incref(consensus_cache_entry_t *ent);
+void consensus_cache_entry_decref(consensus_cache_entry_t *ent);
+
+void consensus_cache_entry_mark_for_removal(consensus_cache_entry_t *ent);
+void consensus_cache_entry_mark_for_aggressive_release(
+                                            consensus_cache_entry_t *ent);
+int consensus_cache_entry_get_body(const consensus_cache_entry_t *ent,
+                                   const uint8_t **body_out,
+                                   size_t *sz_out);
+
+#endif
+