Conversion: Fix a regression in the last release that broke conversion
of a few files with comments just before a chapter start. See #1188635
Merge branch 'master' of https://github.com/cbhaley/calibre
Speed up reading the db

Principally by storing dates as UTC instead of local time. Includes some
micro optimizations in the code paths to build the maps.
Micro-optimization for get_categories()
pep8
Micro-optimize mi.set_identifiers()
...
Search caching for the new backed with a simple LRU cache
El Tribuno Salta and Jujuy by Darko Miletic

Fixes #1203724 [New recipes for El Tribuno Salta and Jujuy](https://bugs.launchpad.net/calibre/+bug/1203724)
Update supported firmware version for Kobo to 2.8.1

No changes necessary except to bump the firmware version.
Small fixes to get_metadata()
Speed up evaluation of composite columns

Use a ProxyMetadata object that lazily evaluates its fields on demand,
thereby avoiding the overhead of get_metadata() on every composite field
evaluation.
Tentative {virtual_libraries} template support

This should allow the use of {virtual_libraries} in custom column
templates, with good performance, thanks to search caching.
Make the new field {virtual_libraries} work.

If it is turned back into a function then we can avoid the changes to __init__.py and field_metadata.py