Massive export speedup

[Kris Deugau]

Instead of regenerating the entire on-disk data file straight from the database each time, export individual zones to separate files in a workspace, then concatenate them. Add a "modified" flag to the domain or revzone so we only have to walk through the details of zones that have been modified, and update those files - should give close to an order of magnitude or better speedup.

Observed on test copies of live data:

• 3081 local forward zones
• 2598 imported records from flatfiles in "main" zone
• 271 imported revzones from flatfiles
• 315 imported forward zones from autoaxfr flatfiles

Initial local forward zones only, ~43-65s
Add all current flatfiles, ~114-160s

[Kris Deugau]

(In [368]) /trunk

First stage implementation of export caching for performance. See #38.
A lot of the actual performance boost comes from indexes on a couple of
columns in WHERE clauses:

• status column on domains and revzones
• type, domain_id and rdns_id columns on records

Net speedup on ~100K records and ~3K zones was
~120s -> 20s (no cache) -> 4s (cache active)

[Kris Deugau]

(In [521]) /trunk

Finally commit a fix for the caching system so that it really
exports things properly. See #38.

[Kris Deugau]

Resolving; numerous tweaks, enhancements, and "OMG-what-was-I-smoking" block rewrites have brought export time down to ~10s even if we set the flag to always refresh the cache.