fix up some TODOs in replica.rs

Cargo.lock

@@ -131,6 +131,12 @@ dependencies = [
  "bitflags",
 ]
 
+[[package]]
+name = "either"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
+
 [[package]]
 name = "failure"
 version = "0.1.6"
@@ -176,6 +182,15 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "itertools"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "284f18f85651fe11e8a991b2adb42cb078325c996ed026d994719efcfca1d54b"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itoa"
 version = "0.4.4"
@@ -607,6 +622,7 @@ dependencies = [
  "chrono",
  "clap",
  "failure",
+ "itertools",
  "kv",
  "lmdb-rkv",
  "proptest",