The skiplist is not concurrent but could be used concurrently

[cfzjywxk]

The arena in skiplist does not support concurrent operations(#116), but it could be used concurrently when allow_concurrent_write is set to true.

Concurrent tests such as bench_read_write_skiplist will crash with [SIGSEGV](signal: 11, SIGSEGV: invalid memory reference) when reducing the arena capacity, for example changing the https://github.com/tikv/agatedb/blob/master/skiplist/benches/bench.rs#L47 size from 512 << 20 to 512.

Supporting the concurrent arena allocator type may be necessary.

[Y-jiji]

It seems like originally you guys wanted to implement thread-local arena for the skiplist but later things went off the rail. Therefore, I suggest in lieu of making the arena concurrent-compatible, we may just utilize the thread_local crate and fulfill the original intention. And maybe I can fix it.