expands lifetime of SlotStats (#23872)

Current slot stats are removed when the slot is full or every 30 seconds
if the slot is before root:
https://github.com/solana-labs/solana/blob/493a8e234/ledger/src/blockstore.rs#L2017-L2027

In order to track if the slot is ultimately marked as dead or rooted and
emit more metrics, this commit expands lifetime of SlotStats while
bounding total size of cache using an LRU eviction policy.

Cargo.lock

@@ -5075,6 +5075,7 @@ version = "1.11.0"
 dependencies = [
  "assert_matches",
  "bincode",
+ "bitflags",
  "byteorder",
  "chrono",
  "chrono-humanize",
@@ -5085,6 +5086,7 @@ dependencies = [
  "lazy_static",
  "libc",
  "log",
+ "lru",
  "matches",
  "num-derive",
  "num-traits",