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.

qemu/unicorn_common.h

@@ -140,6 +140,7 @@ static inline void uc_common_init(struct uc_struct* uc)
     uc->set_tlb = uc_set_tlb;
     uc->memory_mapping = find_memory_mapping;
     uc->memory_filter_subregions = memory_region_filter_subregions;
+    uc->flatview_copy = flatview_copy;
     uc->memory_cow = memory_cow;
 
     if (!uc->release)