Add capacity limit to Recycler / Optimize when assertion is off

Motivation:

- As reported recently [1], Recycler's thread-local object pool has unbounded capacity which is a potential problem.
- It accesses a hash table on each push and pop for debugging purposes.  We don't really need it besides debugging Netty itself.

Modifications:

- Introduced the maxCapacity constructor parameter to Recycler.  The default default maxCapacity is retrieved from the system property whose default is 256K, which should be plenty for most cases.
- Recycler.Stack.map is now created and accessed only when assertion is enabled for Recycler.

Result:

- Recycler does not grow infinitely anymore.
- If assertion is disabled, Recycler should be much faster.

[1] https://github.com/netty/netty/issues/1841