Introduce SyncMode config property

[dceleda]

Suggested by @LukaszRozmej
Could we change to have one property SyncMode with values from enumeration FullSync, FastSync, PivotSync, SnapSync instead of bool flags FastSync, FastBlocks, SnapSync? Those are now exclusive flags and make configuration confusing.

Additionally:
The entire application code should not use old properties.
The properties should be marked as deprecated, setting them should cause a warning to be displayed

[smartprogrammer93]

FullSync, FastSync, PivotSync, SnapSync
Cover almost all cases but what about SnapSync with FastBlock as well? SnapPivotSync maybe?

[smartprogrammer93]

Also, how do we go about being backwards compatible? Meaning, if someone updated nethermind to a newer version using an old config file that has the bool flags.