do not use declarations in the global namespace

[vagetablechicken]

https://github.com/4paradigm/OpenMLDB/blob/5c06b0ff21199fcd8daf23399248ff39f7b3ff61/src/zk/dist_lock.h#L29-L30

Using declarations in the global namespace in headers are prohibited. Move ThreadPool to openmldb::zk space. using ::openmldb::zk::ZkClient; is useless, remove it.

[NevilleMthw]

Is this issue being worked on? If not, do you want something like this: ::openmldb::zk::ThreadPool; with line 30 removed?

[vagetablechicken]

Is this issue being worked on? If not, do you want something like this: ::openmldb::zk::ThreadPool; with line 30 removed?

Nope, just move line 29 in openmldb::zk space, e.g. move to line 34 https://github.com/4paradigm/OpenMLDB/blob/5c06b0ff21199fcd8daf23399248ff39f7b3ff61/src/zk/dist_lock.h#L32-L34 in the namespace, not the global namespace