Set default transaction pool size limit to 100MB.

[aborg-dev]

This PR enables the limit discussed in #3284.

I've considered another approach to rolling this out by changing the value in config.json distributed through S3, but that would require more work both on our side and on validators without adding much benefit.
Specifically, I've checked that we have a good safety margin here, as over the last month on the testnet, the max size of the transaction pool on the validators was < 40 KB: https://nearinc.grafana.net/goto/AhZFN__4R?orgId=1.

@nikurt What would be a good place to document this field for validators? I saw https://near-nodes.io/, but couldn't find the appropriate section there.

[nikurt]

@nikurt What would be a good place to document this field for validators? I saw https://near-nodes.io/, but couldn't find the appropriate section there.

@Akashin At a minimum it should be mentioned in the release notes. Please consider creating a Jira task. We don't have a comprehensive documentation for config.json. One of the best option is to provide human-readable comments in the code.
cc: @marcelo-gonzalez

[aborg-dev]

@nikurt What would be a good place to document this field for validators? I saw https://near-nodes.io/, but couldn't find the appropriate section there.

@Akashin At a minimum it should be mentioned in the release notes. Please consider creating a Jira task. We don't have a comprehensive documentation for config.json. One of the best option is to provide human-readable comments in the code. cc: @marcelo-gonzalez

I've added longer documentation in the field docstring.

I wasn't able to find where exactly I need to create a task in Jira, is this a part of some release process?
We have a tracking bug on GitHub #3284 that is mirrored in Jira: https://pagodaplatform.atlassian.net/browse/DVPT-522 - does it make sense to reuse it? Should I reassign it to the next release engineer? (presumably @marcelo-gonzalez ?)