Create a uid hash for imap. (see #3905)

This hash will allow for more efficient UID SEARCH processing,
replacing a linear scan with a hash lookup.

imap/imap.c

···
 	FREE (&idata->cache[cacheno].path);
       }
 
+      int_hash_delete (idata->uid_hash, HEADER_DATA(h)->uid, h, NULL);
+
       imap_free_header_data ((IMAP_HEADER_DATA**)&h->data);
     }
   }
···
     /* mailbox may not have fully loaded */
     if (ctx->hdrs[i] && ctx->hdrs[i]->data)
       imap_free_header_data ((IMAP_HEADER_DATA**)&(ctx->hdrs[i]->data));
+  hash_destroy (&idata->uid_hash, NULL);
 
   for (i = 0; i < IMAP_CACHE_LEN; i++)
   {

imap/imap_private.h

···
   unsigned char reopen;
   unsigned int newMailCount;
   IMAP_CACHE cache[IMAP_CACHE_LEN];
+  HASH *uid_hash;
   unsigned int uid_validity;
   unsigned int uidnext;
   body_cache_t *bcache;

imap/message.c

···
 static int msg_parse_fetch (IMAP_HEADER* h, char* s);
 static char* msg_parse_flags (IMAP_HEADER* h, char* s);
 
+static void imap_update_context (IMAP_DATA *idata, int oldmsgcount)
+{
+  CONTEXT *ctx;
+  HEADER *h;
+  int msgno;
+
+  ctx = idata->ctx;
+  if (!idata->uid_hash)
+    idata->uid_hash = int_hash_create (MAX (6 * ctx->msgcount / 5, 30));
+
+  for (msgno = oldmsgcount; msgno < ctx->msgcount; msgno++)
+  {
+    h = ctx->hdrs[msgno];
+    int_hash_insert (idata->uid_hash, HEADER_DATA(h)->uid, h, 0);
+  }
+}
+
 /* imap_read_headers:
  * Changed to read many headers instead of just one. It will return the
  * msgno of the last message read. It will return a value other than
···
   {
     mx_alloc_memory(ctx);
     mx_update_context (ctx, ctx->msgcount - oldmsgcount);
+    imap_update_context (idata, oldmsgcount);
   }
 
   idata->reopen |= IMAP_REOPEN_ALLOW;