Fix _processing flag in BaseClient.

[marshall-lee]

I had experienced some problems with flush() in a following scenario:

captureException(error); // Sets _processing=true. Internally, this one is asynchronous because of file I/O
captureMessage('foo');   // Sets _processing=true and immediately sets _processing=false because it's synchronous.
await flush(2000);       // Then, flush doesn't wait for captured error because _processing=false but the message is not even in a buffer yet.
os.exit();               // Finally, I miss that exception in Sentry.

I tried to change fs.readFile to fs.readFileSync in packages/node/src/parsers.ts but file I/O is not the only issue. beforeSend callback could result in asynchronous execution too.

So my proposal is to change _processing from boolean to number counter. Another option would be to introduce another PromiseBuffer and just drain it. What do you think?

[kamilogorek]

I'm totally fine with a solution like that. And thankfully it works out of the box with my todays changes to processEvent 😅
Thanks!