BBlockCache: Use a pthread_mutex instead of BLocker.

The lighter "struct mutex" is in a private header, so we can't
inline it in a public one, so just use pthread_mutex here.

By adjusting padding, the class size stays the same (72 bytes
on 32-bit, 96 bytes on 64-bit; confirmed via static_assert.)

A quick benchmark running "new/delete BMessage" in a loop on 4
threads at once goes from 30-36 seconds before this commit
to around 13-17 seconds, sometimes as low as 3 seconds, afterwards,
so clearly this is a significant improvement.

This also eliminates another BLocker allocated on application startup.

headers/os/support/BlockCache.h

@@ -6,7 +6,7 @@
 #define _BLOCK_CACHE_H
 
 
-#include <Locker.h>
+#include <SupportDefs.h>
 
 
 enum {
@@ -31,16 +31,18 @@ private:
 						BBlockCache	&operator=(const BBlockCache &);
 
 private:
-		struct _FreeBlock;
+	struct _FreeBlock;
 
-		_FreeBlock*	fFreeList;
-		size_t		fBlockSize;
-		int32		fFreeBlocks;
-		int32		fBlockCount;
-		BLocker		fLocker;
-		void*		(*fAlloc)(size_t size);
-		void		(*fFree)(void *pointer);
-		uint32		_reserved[2];
+	_FreeBlock*		fFreeList;
+	const size_t	fBlockSize;
+	void*			(*fAlloc)(size_t size);
+	void			(*fFree)(void *pointer);
+
+	pthread_mutex_t fLock;
+	int32			fFreeBlocks;
+	int32			fBlockCount;
+
+	uint32 _reserved[6];
 };