introduce '+=' operator (atomic increment)

Also kill table_count built-in function, use += instead

Before:
table_count(tbl, key, val)

After:
tbl[key] += val

Remember that:
"tbl[key] += val" is not equal with "tbl[key] = tbl[key] + val",
the first one is more efficient than second one, because first one
just fetch key from table once.

currently '+=' operator only support atomic increment on table
index, not upval and local variable, support those in future.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>