Bug Fix: getExternalFilesDir null check

compatible with RN > 0.49

Fix typo: "Status" instead of "Statu"

Update README with proper status code handling

RN > 0.49 requires to implement requiresMainQueueSetup.
this fix and suppress that yellow box warning

```
Module RNFetchBlob requires main queue setup since it overrides `init` but doesn't implement `requiresMainQueueSetup`. In a future release React Native will default to initializing all native modules on a background thread unless explicitly opted-out of.
```

Synchronized tables, separated requests and network logic and other...

Inform linter that we're handling android api versions correctly

Implement `requiresMainQueueSetup`, fixes #553

Fix README.md

Fix README

We have a crash in our application, and based on our analysis the culprit is a
race condition in RNFetchBlobNetwork initialization.

Crashing thread:

        Crashed: com.apple.root.default-qos
	0  libobjc.A.dylib                0x10416bacb objc_msgSend + 11
	1  Foundation                     0x103d3b644 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 153
	2  Foundation                     0x103c23f8c -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 61
	3  Foundation                     0x103c23f3a -[NSOperationQueue setMaxConcurrentOperationCount:] + 198
	4  prymr                          0x100d579aa -[RNFetchBlobNetwork init] (RNFetchBlobNetwork.m:112)
	5  prymr                          0x100d63a73 __65-[RNFetchBlob fetchBlob:taskId:method:url:headers:body:callback:]_block_invoke (RNFetchBlob.m:131)
	6  prymr                          0x100d6006b __85+[RNFetchBlobReqBuilder buildOctetRequest:taskId:method:url:headers:body:onComplete:]_block_invoke (RNFetchBlobReqBuilder.m:178)
	7  libdispatch.dylib              0x107511585 _dispatch_call_block_and_release + 12

While a second thread is running:

	com.apple.root.default-qos
	0  libsystem_kernel.dylib         0x107891c22 __psynch_mutexwait + 10
	1  libsystem_pthread.dylib        0x1078c6dfa _pthread_mutex_lock_wait + 100
	2  libsystem_pthread.dylib        0x1078c4519 _pthread_mutex_lock_slow + 285
	3  Foundation                     0x103d3b615 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 106
	4  Foundation                     0x103c23f8c -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 61
	5  Foundation                     0x103c23f3a -[NSOperationQueue setMaxConcurrentOperationCount:] + 198
	6  prymr                          0x100d579aa -[RNFetchBlobNetwork init] (RNFetchBlobNetwork.m:112)
	7  prymr                          0x100d63a73 __65-[RNFetchBlob fetchBlob:taskId:method:url:headers:body:callback:]_block_invoke (RNFetchBlob.m:131)
	8  prymr                          0x100d6006b __85+[RNFetchBlobReqBuilder buildOctetRequest:taskId:method:url:headers:body:onComplete:]_block_invoke (RNFetchBlobReqBuilder.m:178)
	9  libdispatch.dylib              0x107511585 _dispatch_call_block_and_release + 12

The patch just adds a dumb double-synchronization to the initialization.