Improve memory configuration #755

[diegosalvi]

This patch solves #755:

• add a new set of configuration to be able to configure memory constraints based on a percentage of maximum memory and not an absolute number: server.memory.data.percentage, server.memory.index.percentage server.memory.pk.percentage. Actually data and pk are simply a configurable value for a previous fixed one. The index is new, see below

• add a new set of configuration to enable indexes / data pages memory separation. Up untill now data and indexes was stored in the same memory space with the same constraint. Now you can configure server.memory.index.percentage (maximum memory percentage value, defaults to 0%) or server.memory.index.limit (byte fixed value, default to 0). If dedicated index memory resolves to a value more than 0 it will be used instead of data memory. By default index will continue to share data memory

• Due to memory inspections I noted that memory occupancy is evaluated enough well, we don't need to keep up to 50% memory free by default for "every else". Increased data default space from 30% to 50%.

• [ x ] I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004

[diegosalvi]

Some test failed, it seems only testing problems, checking them

[eolivelli]

LGTM

[nicoloboschi]

good work!