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Raise default HTTP/2 receive windows and batch HTTP/2 receive-window refills (#481)

* Rename window_size field to send_window_size

The connection and stream structs tracked a `window_size` field for
the client's outbound (send) window and a separately-named
`receive_window_size` field for the inbound window. Renaming the
former to `send_window_size` makes the pair symmetric and removes a
long-standing source of confusion about which direction a bare
`window_size` refers to.

* Raise default HTTP/2 receive windows

Default connection receive window is now 16 MB (was 65_535), sent via
a WINDOW_UPDATE on stream 0 as part of the connection preface. Default
stream receive window is now 4 MB (was 65_535), advertised via
SETTINGS_INITIAL_WINDOW_SIZE in the same preface. Both settable via
the new `:connection_window_size` option and the existing
`:client_settings` option.

Window size / RTT sets a hard cap on per-stream throughput. At the
previous 65_535-byte stream window:

  Path (typical RTT)       | 65 KB    | 4 MB     | 16 MB
  -------------------------|----------|----------|----------
  LAN (1 ms)               | 62 MB/s  | 4 GB/s   | 16 GB/s
  Region (20 ms)           | 3.1 MB/s | 200 MB/s | 800 MB/s
  Cross-country (70 ms)    | 0.9 MB/s | 57 MB/s  | 229 MB/s
  Transatlantic (100 ms)   | 0.6 MB/s | 40 MB/s  | 160 MB/s
  Transpacific (130 ms)    | 0.5 MB/s | 31 MB/s  | 123 MB/s
  Antipodal (230 ms)       | 0.3 MB/s | 17 MB/s  | 70 MB/s

Any caller talking to a server more than a few milliseconds away was
bottlenecked well below their link bandwidth without knowing why. 4 MB
per stream saturates gigabit anywhere on earth; 16 MB at the connection
level lets four streams run in parallel at full rate before the shared
pool binds.

Callers who want the old behaviour can pass `connection_window_size:
65_535` and `client_settings: [initial_window_size: 65_535]` to
`connect/4`.

* Batch HTTP/2 receive-window refills

Previously `refill_client_windows/3` sent ... (continued)

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