Replace @synchronized with NSRecursiveLock

Created by: rgurzhiy

This pull request replaces @synchronized with NSRecursiveLock as in #249 we've discovered that @synchronized holds unretained allocations. The replacement is pretty much straightforward. I've just changed

@synchronized (self.lock) {
...
}

with

[self.lock lock];
...
[self.lock unlock];

and removed all returns in between.

There are still 2 places I'm not sure about: BFTask

- (void)waitUntilFinished {
    if ([NSThread isMainThread]) {
        [self warnOperationOnMainThread];
    }

    [self.lock lock];
    if (self.completed) {
        [self.lock unlock];
        return;
    }
    [self.condition lock];
    [self.lock unlock];
...

BFCancellationTokenRegistration

- (void)dispose {
    [self.lock lock];
    if (self.disposed) {
        [self.lock unlock];
        return;
    }
    self.disposed = YES;
    [self.lock unlock];
...