[Java] Reduce buffer size for ASCII string optimization to 63 bytes

[belugabehr]

As part of my earlier work for AVRO-4074, I introduced a buffer to store strings during serialization. I chose a buffer size of 128 bytes somewhat arbitrarily: it is a power of 2. However, upon further reflection, a value of 63 is a better partition. A string is decomposed into two fields:

a string is encoded as a long followed by that many bytes of UTF-8 encoded character data.

For the binary format of Avro:

int and long values are written using variable-length zig-zag coding.

63 bytes is the largest ASCII string that can be written using only a single byte for the variable-length size. This makes a more sane boundary for the upper limit of this string buffer. With a string size of 128, two bytes are required for the variable length value.