mem2主要用于调试，增加了用于调试的附属结构。使用SQLITE_MEMDEBUG启用。
Each memory allocation looks like this:
** | Title | backtrace pointers | MemBlockHdr | allocation | EndGuard |
** Title:用于描述这段内存，在出错时可以打印出来
backtrace pointer：用于保留调用堆栈
MemBlockHdr:负责这片内存的管理，以及串联unfree的MemBlock
allocation：分配给上层的空间
EndGuard：尾部的哨兵，用于检查内存被踩。还有个“HeadGaurd ”在MemBlockHdr中。
关于backtrace：
1，可以设置malloc的backtrace的深度
2，在生成生成backtrace后可以通过回调函数将其传出去
/ * Open the file indicated and write a log of all unfreed
memory allocations into that log.（这里会打印调用栈） */
void sqlite3MemdebugDump(const char *zFilename)
2，设置哨兵对内存破坏进行检查
/ * Given an allocation, find the MemBlockHdr for that allocation.
**
** This routine checks the guards at either end of the
allocation and if they are incorrect it asserts. */
static struct MemBlockHdr *sqlite3MemsysGetHeader(void
*pAllocation)
3，通过unfree list检查内存泄露
struct MemBlockHdr {
i64 iSize; / Size of this allocation /
struct MemBlockHdr pNext, pPrev; / Linked list of all unfreed memory /