Optimize memory handling (#1963)

* optimize ram block handling

Save the last element of the ram_list. This allows to
faster find where to add new elements when they are not
bigger then page size.

* save ram_list freed

this keeps the optimization for find_ram_offset() intact after snapshot
restore.

* cow only clear the tlb of affected pages

* update flatview when possible

Building each flatview new when the memory has changed is quite
expensive when many MemoryRegions are used. This is an issue when using
snapshots.

* update benchmark for new api

* save flatview in context

this avoids rebuilding the flatview when restore a context.

* init context flatview with zero

* address_space_dispatch_clear remove subpage with higher priority

* docutemnt the options for UC_CTL_CONTEXT_MODE

Specialy stress that with UC_CTL_CONTEXT_MEMORY it is not possible to
use the context with a different unicorn object.

include/qemu.h

@@ -43,6 +43,7 @@ typedef struct {
 typedef struct RAMList {
     bool freed;
     RAMBlock *mru_block;
+    RAMBlock *last_block;
     QLIST_HEAD(, RAMBlock) blocks;
 } RAMList;