Improve the stability of instance creation on a multithreaded environment

[shinnya]

liberasurecode/src/lib.rs

Line 175 in ed45822

// `SIGSEGV` may be raised if encodings are executed (in parallel) immediately after creation.

std::thread::sleep(Duration::from_millis(10)) is not enough and causes build failures on TravisCI. We need a new solution.

See #2 .

[yuezato]

First, we considered this is related to the implementation of Drop.
However, after changing the codes to take a lock in Drop, SIGSEGVs still happen.
yuezato@8f414bd

Indeed, on this commit, we can see the following situation:

$ cargo test
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running target/debug/deps/liberasurecode-0c14855b0b365a2c

running 44 tests
test tests::it_works ... ok
test tests::reconstruct_fails ... ok
test tests::reconstruct_works ... ok
test tests::test0 ... ok
test tests::test1 ... ok
test tests::test10 ... ok
test tests::test11 ... ok
test tests::test12 ... ok
test tests::test13 ... ok
test tests::test14 ... ok
test tests::test15 ... ok
test tests::test16 ... ok
test tests::test17 ... ok
test tests::test18 ... ok
test tests::test19 ... ok
test tests::test2 ... ok
test tests::test20 ... ok
test tests::test21 ... ok
test tests::test22 ... ok
error: process didn't exit successfully: `/Users/yuuya_uezato/public_frugalos/fork/liberasurecode/target/debug/deps/liberasurecode-0c14855b0b365a2c`
(signal: 11, SIGSEGV: invalid memory reference)

Consequently, we decide to make thread-safe versions of backends of openstack/liberasurecode.