Fix crash with race condition of playerPool

[robinxb]

There is a race condition crash with issue #58.
I added synchronized scope with operations of [self playerPool]
Since ReactNative's bridge always executes codes in asynchronous mode, this should fix the issue.

[trepidity]

Nice!
Have you tested this in your app?

[robinxb]

Yes I've tested this in our app.

[trepidity]

I've merged this into unstable.
I will release it at the end of the month.

[robinxb]

May I ask what's the date of releasing new version? :)

[almostintuitive]

We're also waiting for this fix :)

[almostintuitive]

This is currently our 2nd most frequent crash: https://sentry.io/share/issue/e4075aec0f8b44d5b48e8d94492f2715/

[paulmelnikow]

#476 was about fixing a key value, but also made some changes related to these, which involve running @synchronized(self) around releasing. I think these changes can be made additively to those, though I also want to be careful of deadlocks. I wonder if it would be better to centralize on a single lock object (self) rather than using both.