Reduce the overall memory usage of the LwM2M library

[zephyrbot]

Reported by Michael Scott:

Currently, the LwM2M library has some fairly high memory usage:

• Several stacks are created across various portions of the library. Perhaps we can merge a few of them together or examine optimal sizes.
• 2 sets of pendings and reply arrays are created where once some refactoring is done, we can probably reduce this to 1 array using a single CONFIG item
• And there are probably other places where memory can be trimmed down.

(Imported from Jira ZEP-2418)

[mike-scott]

#4554 reduces flash size by about 3.5k if printf functions are added by user samples.

[mike-scott]

Closing this one as a significant amount of memory has been reclaimed from the client. Can reopen with a more specific issue later if needed.

[mike-scott]

See: #7394