Code review comment for lp://staging/~diego-fmpwizard/drizzle/bug-fixes

* Fixed bug* #377826 "Failure to detect error in allocating transcation_prealloc_size"

* I converted transaction_prealloc_size to a uint64_t.

That eliminated the strange values that ended up assigned to "transcation_prealloc_size" when you tried to set it to a value of 1024*1024*1024*4 or higher.

* To get a warning on those same cases, I had to add:

throw_bounds_warning(session, true, true, getName(), (int64_t) tmp);

to

set_var.cc:

bool sys_var_session_uint64_t::update(Session *session, set_var *var)

* I also enabled the test suite under big/t/variables-big.test
* Had to modify a test case for
    set @@max_heap_table_size= 4294967296;
  ** Because it now returns a warning about the value being truncated (truncated to 4294966272 ) (This is different than what MySQL does, MySQL accepts a value of 4294967296)