Remove contract event topic limits

[jayz22]

We have a few limits in the contract event topics:

• No. topics <= 4
• Topic byte size < 64

These were some arbitrary limits put there initially, but are no longer needed. They are also not enforced consistently (diagnostic event contains a ContractEvent but we don't check that limit). Contract events are hashed into the transaction result, both contract and diagnostic contract events are included in the meta and is subject to limits and fees.

[jayz22]

On second thought, these limits should not apply to diagnostic events (the current behavior is correct), since diagnostic events should be infallible (see #962 to validate that).

[jayz22]

I think it is necessary to more clearly spell out the event limits as network config parameters.

• number of topics
• single topic size limit
• data size limit

Without these limits, contracts may DOS the downstream system by submitting many large events (each up to the metadata size limit).

They need to be specified in the CAP (stellar/stellar-protocol#1371) and the host need to adapt to them.

[sisuresh]

The 4 limit was chosen arbitrarily since that's the limit of topics in the evm. We don't need to follow that though because the topics are not indexed in the protocol like in Ethereum.

[jayz22]

Based on recent discussions, downstream are more concerned with the meta data size to maintain than the number of topics. Thus we've decided to move the topics limit.
The meta size limit has been repurposed to event size limit in https://github.com/stellar/stellar-xdr/pull/127/files, and will be set to a much lower value ~10k to limit the downstream impact.