Implement chunk server state update handling

Fix how chunk servers handle state updates from the metadata server
based on the design in DESIGN.txt. The implementation now:

1. Checks if chunks in the add_list exist in the chunk store and
   collects any missing chunks
2. Unmarks chunks in the add_list that were previously marked for removal
3. Marks chunks in the rem_list for removal with timestamps
4. Responds with SUCCESS if all chunks are present, or ERROR with the
   list of missing chunks

Added:
- RemovalList data structure to track chunks marked for deletion with
  timestamps
- chunk_store_exists() to check if a chunk file exists
- Helper functions for removal list management (init, free, add, remove, find)

This implements the garbage collection mechanism described in DESIGN.txt
where chunks are marked for removal and can be unmarked if they appear
in the add_list again.

src/chunk_server.h

@@ -25,6 +25,17 @@ typedef struct {
     PendingDownload *items;
 } PendingDownloadList;
 
+typedef struct {
+    SHA256 hash;
+    Time   marked_time;
+} PendingRemoval;
+
+typedef struct {
+    int count;
+    int capacity;
+    PendingRemoval *items;
+} RemovalList;
+
 typedef struct {
     bool       trace;
     Address    local_addr;
@@ -34,6 +45,7 @@ typedef struct {
     ChunkStore store;
     bool downloading;
     PendingDownloadList pending_download_list;
+    RemovalList removal_list;
 } ChunkServer;
 
 int chunk_server_init(ChunkServer *state, int argc, char **argv, void **contexts, struct pollfd *polled, int *timeout);