Add version of atomicModifyWithLens discarding the result
Add version of atomicModifyWithLens discarding the result

In some cases, we only care about a change being made atomically
to an IORef, but do not want to simultaneously read a value from
it. In this case, we can accept a function only returning the
new value, and not a pair of result and new value.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,leshchevds/ganeti,bitemyapp/ganeti,andir/ganeti,onponomarev/ganeti,bitemyapp/ganeti,ganeti/ganeti,andir/ganeti,andir/ganeti,onponomarev/ganeti,ganeti/ganeti,leshchevds/ganeti,mbakke/ganeti,leshchevds/ganeti