Hide 'stanza reconstruction' behind configuration

[guusdk]

When only a message body, and not the original stanza, is persisted, the plugin attempts to reconstruct a stanza. This is a suboptimal solution, as data will have been lost. MAM results based on this reconstruction will be imprecise, which can lead to bugs.

This feature should be made configurable, and turned off by default.

Note that for personal archives, stanzas are now always persisted if message archiving is enabled (this might not have been the case in the past - old databases might have null values in the stanza column). For MUC archives, stanza persistence is controlled by an additional flag. (This inconsistency should be removed. See #116 )

[Fishbowler]

Perhaps less suboptimal following changes made in #140