inodes: preserve access time while unloaded

Summary:
Retain `lastFsRequestTime` in `UnloadedInode` and restore it when `InodeMap` reconstructs an `InodeBase`. Keep restoration private to `InodeMap` so a published inode cannot overwrite a newer filesystem request timestamp. Serialize the raw timestamp in an optional takeover field so inode age survives graceful restart; payloads from older versions default to the receiver current time.

I am making GC more aware/compatible with remembered inodes. Previously GC would load remembered inodes and default the last fs time to "now" so they would never get GC'd. Instead we track the last fs time for remembered inodes in general and across takeover.

Reviewed By: MichaelCuevas

Differential Revision: D115455900

fbshipit-source-id: 4a7240fd2baaee68e19c6bd4228bc4dfbfb742f1

Author: muirdm

third-party/watchman/src/eden/fs/takeover/takeover.thrift

@@ -37,6 +37,8 @@ struct SerializedInodeMapEntry {
   // string as a valid ID.
   6: optional string hash;
   7: i32 mode;
+  // EdenTimestamp persistent wire encoding.
+  8: optional ui64 lastFsRequestTime;
 }
 
 struct SerializedInodeMap {