Delete values immediately from DB overlay

[arkpar]

About 20% block import speed increase

[coveralls]

Coverage increased (+0.05%) to 76.421% when pulling c145764 on db-purge-fix into d14b687 on master.

[rphmeier]

This doesn't add entries of the form (key, -1) like remove did when there is no entry for the key

[gavofyork]

tests shouldn't be passing; this needs fixing and a test to ensure it's caught.

[arkpar]

OverlayRecentDB does not keep deleted key-values in memory. All calls to remove were followed by purge. This logic did not change with this PR

[arkpar]

This is a pruning overlay for the journaled states, not to be confused with transaction overlay.

[gavofyork]

ok, however, the function name should change (since it's not strictly equivalent to remove + purge), the documentation should be made abundantly clear that it has subtly different semantics (since this could very easily introduce very difficult to detect bugs if a later accidental replacement were to happen that depended on remove_and_purge + insert giving a 0 ref-count) and it should be demonstrated at the call site that the refs is always > 0.

there should also be an assertion to the same effect in the function.

[arkpar]

How is it not equivalent? The function does what the name says, removes a reference count and purges it if it reaches zero or negative.

remove();
purge();
insert();

is equivalent to

remove_and_purge();
insert();

for a particular key of course

[gavofyork]

If the key is not in the DB at the beginning, the semantics are different.

• remove will insert a key of refs -1 (L238);
• purge will leave it in there (L119);
• insert will increase it back to 0.

whereas:

• remove_and_purge will do nothing;
• insert will insert with a reference count of 1.

[arkpar]

I see, will change remove_and_purge to match purge behaviour and add a test

[coveralls]

Coverage increased (+0.03%) to 76.403% when pulling 351fadc on db-purge-fix into d14b687 on master.

[coveralls]

Coverage decreased (-0.02%) to 76.355% when pulling 351fadc on db-purge-fix into d14b687 on master.