Fix websocket bug causing SHiP to stop serving blocks

[porkchop]

Description

A known bug in < 4.x SHiP nodes causes them to stop sending blocks. See: AntelopeIO/leap#1358 (comment)

No fix is scheduled by the Antelope team for 5.x. It would be good to have this addressed for 5.x versions.

For reference, the issue was solved for Spring here: AntelopeIO/spring#237

Thoughts, comments and offers are welcome!

[rakeden]

"The issue is a mutex introduced in 3.2, it can be fixed without a full rewrite"