fix: ws race and leak fixes

[sonicfromnewyoke]

Problem

ws client has a couple of concurency bugs:

• goroutine and chan leaks on conn drop
• no sync on the closed flag
• Close() returning before background goroutines finished
• Response() methods that leaked a goroutine per call. For example. WaitForConfirmation method leaks in the loop

Summary of Changes

• added a per-subscription mutex to protect channel sends and closes
• removed the fundamentally broken Response() and RecvWithTimeout() methods from all subscription wrappers, replacing their only caller with Recv + context timeout
• added WaitGroup to Close so callers can rely on cleanup being complete
• reduced channel buffers from 200K/100K to 200/1, which made error sends non-blocking to prevent deadlocks
• extended unit tests to cover all the cases above with -race flag

closes #327
closes #334
closes #270