BREAKING: New ILP/HTTP and Sender.from_conf API

[amunra]

Overview

• Implement Sender.from_conf and Sender.from_env APIs which will simplify construction of the sender.
• Make at= argument in .row() and .dataframe() methods mandatory.
• Support the latest version of Pandas.
• Numerous documentation improvements.

Tasks

• wire up new c-questdb-client code.
• get HTTP basics working.
• Match up new config string logic: https://docs.google.com/document/d/1EQZjYrJO_Ll8RrSIZpbr15Ul3N-xAtTxvrunI-TpOR8/edit
• Rework params to match conf args.
• implement row-count and interval based auto-flushing.
• make at=.. KW parameter required in row and dataframe API, new "ServerNow" for old at=None behaviour.
• implement Sender::from_conf('...', **kwargs).
• Implement with sender.transaction() to temporarily disable auto-flushing.
• Implement sender.flush(buffer, transactional=True).
• test auto_flush_interval
• test auto_flush_rows
• test auto_flush='off' and with sender: block: Final flush should still happen.
• test sender.transaction().
• set user agent in sender opts.
• test and implement init_buf_size and max_buf_size params.
• test every single param as python type.
• test every single param as str equivalent (e.g. 'on'/'off' for bool)
• Disable _FLUSH_FMT for HTTP.
• review all ctor params have correct defaults.
• update existing examples
• write a new from_conf example and register it with the manifest file
• test with dataframes and http, including transactions
• update API docs
• write HTTP intro docs
• fix auto-flushing comments in examples
• change auto-flush default for TCP to 600 rows.
• test calling txn.commit() multiple times.
• test calling sender.flush() from within a txn block.

Closes #51.
Closes #62.

[amunra]

For the related Rust/C client changes, see questdb/c-questdb-client#50 and questdb/c-questdb-client#54

[nwoolmer]

Overall LGTM

[goodroot]

Commented in a few areas.

Please @goodroot anywhere you'd like me to pickup next week.

Docs look OK to me on surface, prefer to unblock the API changes.