esp8266: Fix lost chars problem when block-xfering data (e.g., when pasting).
Pasting more or less sizable text into ESP8266 REPL leads to random chars missing in the received input. Apparent cause is that using RTOS messages to pass individual chars one by one is to slow and leads to UART FIFO overflow. So, instead of passing chars one by one, use RTOS msg to signal that input data is available in FIFO, and then let task handler to read data directly from FIFO. With this change, lost chars problem is gone, but the pasted text is truncated after some position. At least 500 chars can be pasted reliably (at 115200 baud), but 1K never pastes completely.