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

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