Provide safety mechanisms for working with thread-locals

[qwwdfsad]

Currently, kotlinx.coroutines provides primitives to work with thread locals.

The problem is that they are not quite safe and it is easy to accidentally forget them (#1016).
Moreover, sometimes it is necessary to know whether thread local is present in the context in order to do some action (e.g. probabilistic request tracing using thread locals, the case from Circlet).

In all cases, it is impossible to check whether thread local element is actually present in the context, and checking thread local itself is not an option, because it can have a stale or default value.

We can provide following extensions (similar to coroutineContext) to cover that cases and promote their usages in the documentation:

suspend fun TheadLocal<*>.ensurePresent(): Unit
suspend fun TheadLocal<*>.isPresent(): Boolean
suspend inline fun <T> ThreadLocal<T>.ifPresent(block: suspend (T) -> Unit) // Or R?

[glebleonov]

suspend inline fun <T> ThreadLocal<T>.ifPresent(block: suspend (T) -> R): R? would be better since one can use it like the following:

suspend fun <T> runWithMyThreadLocal(f: () -> T): T {
    return myThreadLocal.ifPresent {
        it.doSomethingWith(f())    
    } ?: f()
}