Remove the dependence of curve-nbd, fio, and tgt on nebd

[ilixiaocui]

Is your feature request related to a problem? (你需要的功能是否与某个问题有关?)

和性能相关。当前curve-nbd、tgt都依赖于 nebd。之前是为了客户端可以做热升级，当前curve块存储的客户端已经是稳定版本，为了提升性能，希望将nebd模块去掉，上层应用直接对接curve-client sdk。

The issue related to performance. At that time curve-nbd, tgt all depended on nebd. Previously, it was for the hot upgrade of the client. At that time, the client of the curve block storage was already a stable version. In order to improve the performance, it was hoped that the nebd module would be removed.The upper application directly use curve-client sdk.

Describe the solution you'd like (描述你期望的解决方法)

Describe alternatives you've considered (描述你想到的折衷方案)

Additional context/screenshots (更多上下文/截图)

[ilixiaocui]

now curve-nbd，fio，tgt all support curve-sdk directly.

[legionxiong]

now curve-nbd，fio，tgt all support curve-sdk directly.

Are there any code/pr links?