当通过task 插入多条数据时报错。
DBcontext 不是线程安全的， 如果是WebAPI 为每个请求创建DbContext
部分解释：
http://stackoverflow.com/questions/28106267/insert-into-dbcontext-multithreading#
DbContext isn‘t thread-safe.
One context and many threads - it is a bad practice. It is best to create the context for each thread. I could recommend to you reconsider your algorithm and reate the context for each thread
EF 多线程插入 Insert into DbContext Multithreading
原文：http://www.cnblogs.com/miralce/p/4720025.html