Smart `IR` -> `SN` GAS transfers

After #873 there is rude notary deposit logic: SN checks its notary and GAS balances and tries to keep 1:3 GAS to Notary balance proportion with GAS/2 OR 1 deposits.

But it could not be always good, e.g. for tests, it is common practice to set epoch to 20 blocks in dev-env, but IR sends GAS to SN only once in 300 blocks(it does not depends on epoch duration), so SN runs out of GAS(not notary balance) with the time.

Questions

1. How often/much should IR send GAS to SN?
2. Should it depend on netmap size/SN's balances?
3. Should SN change its deposit logic?