Fix deadlock when connection closed

[jiazhai]

Fixes #366

Motivation

In current code of pulsar/internal/connection.go we have 2 channels, closeCh and incomingRequestsCh. when the connection closes, the current mis-use of these 2 channels may have a deadlock.
PR #366 has detailed steps to reproduce and the root cause analysis .
This PR tries to fix the deadlock.

Modifications

• make the close logic independent, not in the same loop of normal events handling.
• when the connection closed, handle the existing requests in the channel and return an error to avoid deadlock.

Verifying this change

passed the tests in #366
current ut passed

[wolfstudy]

Cool, LGTM+1

[jiazhai]

Thanks @wolfstudy for the help