Merge fb-prod201903

1. Update rocksdb submodule to 8a1ecd1982341cfe073924d36717e11446cbe492 Fix build failures due to missing JEMALLOC_CXX_THROW macro (#5053) (v5.8-1365-g8a1ecd198)

2. Add new MyRocks variables "read_free_rpl" and "enable_insert_with_update_caching".

3. Cherry-pick the following commits from `fb-prod201903`:

• da9bd74f8d5752cf83a84a655a6d3eb38152180c Added a new variable to control read-free replication
• 18b026729d68099f50aae9b40413b871646bc229 re-record the result of mtr test rocksdb.show_engine
• c470e31144ffec8ba228b4272ce378e2ed4192d1 Update key fields encode/decode into static func
• e1516e7601e5a721459ab38054a8d75091736f41 Add new class Rdb_converter to expose decode API
• 77032004ad23d21a4c386f8136ecfbb071ea42d6 Simplify TTL value fetch for ttl column
• ab742e469d73f42251706a078c6b4506b526b3f1 Add encode value slice into Rdb_converter
• a6d547e4e6ac3e642173c20b5dc990df829006c9 Check thd flags during query processing
• ef4e62a6b04ca15eac95627210fcb51336e4fcf5 Add iterator class for decoding value slice
• 469134bea110a7f1c888eb6ebbe4bce5a85b69be Avoid 2nd GetForUpdate on INSERT on DUPLICATE KEY. Improves performance up to 12.98%.

4. Null cherry-picked upstream commits:

• 51dbcc8da12bd28eb1510551eb228c82490cd23c Modifying histogram label for last bucket
• 309f324ea3238006e9f2c0cadfc62b93de461dae Add rocksdb_bulk_load_allow_sk option in mysqldump - this enables bulk loading secondary keys in mysqldump output
• f7d9a8dd3772212458c428924f04e5dcf9ddd8e0 Bug#84958 - InnoDB's MVCC has O(N^2) behaviors (#975)
• 927849171bf264dfb865bcfc7b586c51bc6722c0 Fix the gtid used during idempotent recovery
• 51a625078b58ef2d17ac1ce78af40bb30a9729b5 Sync transaction when number of keys exceed a threshold
• 75017433daa20546f9a71f4590e2ee337efed0a7 Optimize replace into to do a blind write
• 98b0d40af27051ded465a5ca81fac8e3110cc2c1 Fix error handling in group by plans
• 7f81d6f82f3ec68267527a2fd6e774a79deae2b0 Backport HA_ERR_QUERY_INTERRUPTED to 5.6
• 705104c1fd4a536d90f0fec481b06165ef41eea6 Added -DZSTD_STATIC_LINKING_ONLY option
• 0728f653551aa7784bfc780bbe8616e5f7bc12b9 Fix a optimized replace into test.
• d2329cc28b64dab090a2358662faf87a66d9520b Handle column type conversion during key parsing