Better constraints on time machine space allocation

[novoj]

Currently we only have one configuration option for Time Machine data retention in relation to WAL:

• long walFileSizeBytes
• int walFileCountKept

Unfortunately this is not enough - due to the regular vacuuming process (this refers to #760) the size occupied by time travel data can grow quickly and uncontrollably. The process should allow to limit the time travel machine by a multiplier of the size of the active data. I.e. if the current set of file data occupies 1GB, we should be able to specify that the time travel machine data should not exceed 3 times the active size (i.e. 4GB in total), or we could also specify a hard limit in terms of absolute size (whichever is reached first).

This change will require a change in the logic of how we maintain and detect the first timestamp from which the time travel backup is possible (we would probably need to read catalogue headers from the bootstrap and check file existences for the data).