hardening async_key_value_source

[bryanseay]

There was a crash seen in async_key_value_source::lookup_delayed where an invalid row was added to the table. After hardening the class to be explicit about when rows were added, the problem went away. The suspicion is that the lack of a copy constructor occasionally caused issues when inserting via operator[].

[krishnan-ramkumar]

LGTM -- Always work with iterators and never use [] for a lookup or modification is what I have learnt from experience.
m_value_map[key].m_available = false --> eeeek. So wrong on so many levels.

[gnosek]

Can't say I grok the patch enough to pinpoint the "here is the bug" line in the diff but if it helps...