Adding a reverse scan API for raw client

[rahulrane50]

Issue : #440

Problem statement :
Adding a support for reverse scan public API in raw rust client.

Solution :

1. Added a new public API in raw client.
2. Add flag all the way through until new raw scan request is created. The tikv service scan API already has a field for reverse flag. Set it properly based on what caller is passing.

Tests :

cargo build
cargo clippy && cargo fmt
cargo test


PD_ADDRS="127.0.0.1:2379" cargo test --package tikv-client --test integration_tests --features integration-tests
   Compiling tikv-client v0.3.0 (/Users/rrane/work/spiral-repos/client-rust)
    Finished test [unoptimized + debuginfo] target(s) in 10.41s
     Running tests/integration_tests.rs (target/debug/deps/integration_tests-e4cb8a4e124a5c59)

running 26 tests
test raw_bank_transfer ... ok
test raw_cas ... ok
test raw_req ... ok
test raw_ttl ... ok
test raw_write_million ... ok
test txn_bank_transfer ... ok
test txn_batch_mutate_optimistic ... ok
test txn_batch_mutate_pessimistic ... ok
test txn_crud ... ok
test txn_get_for_update ... ok
test txn_get_timestamp ... ok
test txn_insert_duplicate_keys ... ok
test txn_key_exists ... ok
test txn_lock_keys ... ok
test txn_lock_keys_error_handle ... ok
test txn_pessimistic ... ok
test txn_pessimistic_delete ... ok
test txn_pessimistic_heartbeat ... ok
test txn_pessimistic_rollback ... ok
test txn_read ... ok
test txn_scan ... ok
test txn_scan_reverse ... ok
test txn_scan_reverse_multi_regions ... ok
test txn_split_batch ... ok
test txn_unsafe_destroy_range ... ok
test txn_update_safepoint ... ok

test result: ok. 26 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 59.34s

[rahulrane50]

CI integration test failure - Not sure if this is flaky test but it passes locally.

PD_ADDRS="127.0.0.1:2379" cargo test --package tikv-client --test integration_tests --features integration-tests

     Running tests/integration_tests.rs (target/debug/deps/integration_tests-e4cb8a4e124a5c59)

running 26 tests
test raw_bank_transfer ... ok
test txn_batch_mutate_pessimistic ... ok
test txn_crud ... ok
test txn_bank_transfer ... ok
test raw_req ... ok
test txn_batch_mutate_optimistic ... ok
test txn_get_for_update ... ok
test raw_write_million ... ok
test raw_cas ... ok
test raw_ttl ... ok
test txn_get_timestamp ... ok
test txn_insert_duplicate_keys ... ok
test txn_key_exists ... ok
test txn_lock_keys ... ok
test txn_lock_keys_error_handle ... ok
test txn_pessimistic ... ok
test txn_pessimistic_delete ... ok
test txn_pessimistic_heartbeat ... ok
test txn_pessimistic_rollback ... ok
test txn_read ... ok
test txn_scan ... ok
test txn_scan_reverse ... ok
test txn_scan_reverse_multi_regions ... ok
test txn_split_batch ... ok
test txn_unsafe_destroy_range ... ok
test txn_update_safepoint ... ok

test result: ok. 26 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 59.44s

[rahulrane50]

cc @ekexium

[rahulrane50]

@ekexium can you take another look please?

[pingyu]

• Suggest to also add a scan_keys_reverse method for convenience.
• Suggest to add integration test for the scene of reverse scan across regions. Please refer to Fix reverse scan for scene of multiple regions #438.
• Could you kindly remove the comment of "TODO" here as well ? Thanks.

Rest LGTM~

[pingyu]

LGTM~

[rahulrane50]

Thanks for the review and approval @pingyu. can you please help me with merging?

[pingyu]

Thanks for the review and approval @pingyu. can you please help me with merging?

Done. Thanks for your contribution ~