Store message metadata, avoid parsing messages on get

Cargo.lock

@@ -2482,6 +2482,7 @@ dependencies = [
  "hyper 1.0.0-rc.4",
  "hyper-util",
  "jmap_proto",
+ "lz4_flex",
  "mail-builder",
  "mail-parser",
  "mail-send",
@@ -2771,6 +2772,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "lz4_flex"
+version = "0.11.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3ea9b256699eda7b0387ffbc776dd625e28bde3918446381781245b7a50349d8"
+dependencies = [
+ "twox-hash",
+]
+
 [[package]]
 name = "mail-auth"
 version = "0.3.6"
@@ -5757,6 +5767,16 @@ dependencies = [
  "cipher 0.4.4",
 ]
 
+[[package]]
+name = "twox-hash"
+version = "1.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "97fee6b57c6a41524a810daee9286c02d7752c4253064d0b05472833a438f675"
+dependencies = [
+ "cfg-if",
+ "static_assertions",
+]
+
 [[package]]
 name = "typenum"
 version = "1.17.0"