Move signing keys selection to NodeOperatorsRegistry

[skozin]

Currently, signing keys are selected from eligible Node Operators in the following manner:

1. Lido obtains all Node Operators' data (except pubkeys/signatures) from NodeOperatorsRegistry and caches it into memory.
2. Lido performs as many 32 ETH deposits as possible. Before making each deposit, Lido selects the Node Operator with the least effective stake and retrieves its next unused pubkey and signature from NodeOperatorsRegistry.
3. After performing all deposits, Lido updates used keys counts in NodeOperatorsRegistry.

This PR changes the process to the following:

1. Lido calculates the number of deposits it's allowed to make.
2. Lido requests the desired number of unused pubkeys/signatures from NodeOperatorsRegistry, which selects the operators with the least effective stake and returns their pubkeys/signatures as two concatenated byte arrays, updating operators' used keys counts at the same time.
3. Lido performs the deposits.

The new approach has a number of benefits:

1. It uses significantly less gas due to making fewer Aragon-proxied calls and transmitting less data.
2. The resulting code is less coupled and more testable (e.g. one can test pubkey selection without making actual deposits).
3. The size of the Lido contract's bytecode and sourcecode is decreased.

Gas profiling

The profiling was performed for 10 Node Operators, each having one key already used. The numbers below were obtained using the mock deposit contract and corrected for the real deposit gas usage (assuming it's 49000 for inter-contract calls).

	Total gas, 1 deposit	Total gas, 10 deposits
master	517,399	1,349,282
this PR	186,150	985,423
Reduction	64%	26%

Index underflow fix

Also, this PR fixes #227 which was discovered in the process the refactoring.