avoid browser cache when invoking servlet
add a postgresql config and little tuning for sqlite and mysql
add user statistics reporting capability
try to generate users such that some percent of watches to be mutual
(users watch each other) - chunchun.cache.init.mutual.watches.percent
The default is 95% but to avoid slow cache initialization it is done in
one pass and the target percent as well total watches count fluctuates.
End effect is that some users have lower count of watches and many have
less mutual watches than specified.
much simpler and more accurate algorithm for generating mutually
watching users
make sure added followers/following are not duplicates and not this user
use WAL journal_mode for sqlite for improved write (and overall)
performance