Drop all use of OSSpinLock

Apple engineers have pointed out that OSSpinLocks are vulnerable to live locking
on iOS in cases of priority inversion:
. http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
. https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html

- Use a dispatch_semaphore_t within the extension registry.
- Use a dispatch_semaphore_t for protecting autocreation within messages.
- Drop the custom/internal GPBString class since we don't have really good
  numbers to judge the locking replacements and it isn't required. We can
  always bring it back with real data in the future.

objectivec/GPBUtilities.m

diff --git a/objectivec/GPBUtilities.m b/objectivec/GPBUtilities.m
index 5ee6123..eaa28bb 100644
--- a/objectivec/GPBUtilities.m
+++ b/objectivec/GPBUtilities.m
@@ -411,7 +411,7 @@
     return field.defaultValue.valueMessage;
   }
 
-  OSSpinLockLock(&self->readOnlyMutex_);
+  dispatch_semaphore_wait(self->readOnlySemaphore_, DISPATCH_TIME_FOREVER);
   GPBMessage *result = GPBGetObjectIvarWithFieldNoAutocreate(self, field);
   if (!result) {
     // For non repeated messages, create the object, set it and return it.
@@ -420,7 +420,7 @@
     result = GPBCreateMessageWithAutocreator(field.msgClass, self, field);
     GPBSetAutocreatedRetainedObjectIvarWithField(self, field, result);
   }
-  OSSpinLockUnlock(&self->readOnlyMutex_);
+  dispatch_semaphore_signal(self->readOnlySemaphore_);
   return result;
 }