Use less memtables for the entire db, reserved for switch memtable

[ofriedma]

Currently for each CF, we reserve a spare memtable waiting to replace the active one, we could reserve it for the entire db so we will be able to save some memory

[udi-speedb]

@ofriedma -

1. I assume this issue is applicable to Speedb's memtables only. Am I right?
2. How much memory does a single empty memtable consume?

[ofriedma]

1. No, for all implementations
2. Didn't investigate it has it's still under future to research - it depends on the memtable itself

[ayulas]

its completely not correct. this was the propose of doing the global switch #296
the prepare switch memtable is done on a memtable factory, and only on a factory that support that - e.g. enable_switch_memtable_ is true. so it means that if all the cfs use the same factory ,this is on entire db....
currently only hash spdb memtable uses it