Merge branch 'dconeybe/snappy' of https://github.com/firebase/firebase-cpp-sdk into dconeybe/snappy

Author: jonsimantov

cmake/external/firestore_snappy.patch.txt

@@ -42,7 +42,7 @@ index b71a77535..2556d7041 100644
  endif()
 
 -set(version 1.22)
-+set(version e0d5f83a4f80060fe5b5d80025f0ad049bca430e)
++set(version 1.23)
 +
 +ExternalProject_Get_property(snappy SOURCE_DIR)
 +set(snappy_source_dir "${SOURCE_DIR}")

cmake/external/leveldb.cmake

@@ -18,7 +18,7 @@ if(TARGET leveldb)
   return()
 endif()
 
-set(version e0d5f83a4f80060fe5b5d80025f0ad049bca430e)
+set(version 1.23)
 
 ExternalProject_Add(
   leveldb

release_build_files/readme.md

@@ -569,6 +569,8 @@ code.
 ## Release Notes
 ### Upcoming Release
 -   Changes
+    -   Firestore/Database (Desktop): Upgrade LevelDb dependency to 1.23
+        ([#886](https://github.com/firebase/firebase-cpp-sdk/pull/886)).
     -   Firestore (desktop): Enabled Snappy compression support in LevelDb
         ([#885](https://github.com/firebase/firebase-cpp-sdk/pull/885)).