internal/client: remove the lock for idle connection recycle

[tiancaiamao]

After mock testing, I find #270 can not sufficient to solve the problem.
To my surprise, RLock can block RLock in some case

The final decision is to reduce the granularity of the lock.
Just lock the modification of the map...

There is a risk that an idle connection become active again, since recycleMu is removed to protect such case.
In very rare case, someone is maybe using the connection and the connection is closed by the recycle operation.

I think the outcome is acceptable, comparing to the recycle operation blocks the SendRequest() function.

[tiancaiamao]

Damn ... the DCO check ... I open a new one #278

[disksing]

Damn ... the DCO check ... I open a new one #278

you can reset + commit with DCO + push -f
you can also run this script + push -f