proxy: added multithreaded worker support for proxy plugins

[gautampunhani]

This applies the same code change brought in for making output plugins performant in v1.7.0 to the proxy plugins of Go.
Without this change if the go output plugin gets slow/unresponsive, other input plugins and output plugins also were getting stuck because of single-threaded nature of invocation.

This addresses issue fluent-bit-go#45

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change
• [N/A] Attached Valgrind output that shows no leaks or memory corruption was found
  Please note that, Valgrind with golang plugins show leaks without this change as well

Documentation

• [N/A] Documentation required for this feature (Documentation will be same as any output plugin with workers)

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.