#1665 Improve caching

[Marc Delisle]

We already have a caching mechanism (see PMA_Util::cache* functions) which is used to keep some temporary user choices, and some info from the MySQL server that we assume does not change (character sets). This mechanism is currently session-based only.

This RFE is in two parts:

1. Identify other info that we are polling from the MySQL server, which would benefit from caching and implement caching on this info (if it's not too bulky).

2. Verify the feasibility of adding optional memcached support to our caching mechanism.

These parts are related. Indeed, if the new info identified as being cacheable is too bulky, it could be marked as being cacheable only in memcached.

[Madhura Jayaratne]

Added caching for 'lower_case_table_names'. https://github.com/phpmyadmin/phpmyadmin/commit/114581e97f18eaab8252cb4f03d74fd8b7f4faae

[Marc Delisle]

No longer sure that there would be a performance gain by moving caching from session to memcached.