This commit adds support for "prefix keys". Prefix keys differ from
regular tombstone key entries in that the key of the entry should act as
a prefix that matches all series with the same prefix key for the given
time range.
This means only one entry is needed to delete many series.
The tombstone entries now have a maximum length of 16777215 (24 bits),
with the remaining 8 high bits available for setting further options /
meta information about the tombstone entry.
In this case, the top bit is used to indicate that the tombstone entry
is intended to be a prefix. This leaves 7 spare bits for future use.
Jeff Wendling
Edd Robinson
Chris Goller