HUE-2122 [impala] Smarter db and table cache

Cached the db list too
Bumped cached time expiration to 7 days
[metastore] Support of UTF-8 in the comments and names of tables

if value.contains(chinese):
    print "'ascii' codec can't decode byte 0xe4 in position 0: ordinal not in range(128)"
HUE-2076 [core] Create retina icons/logos

Retinified all the logos and images