Limit initial allocated block size by `ByteArrayBuilder` to max block size

[cowtowncoder]

Although increase of block sizes is capped to 128k by ByteArrayBuilder, initial block size is not limited.
Databind used to try to allocate full block for base64 decoding, and due to miscalculation in an earlier version (up 2.9.x), was vastly overallocating, leading to potential failures.
But since builder is supposed to segment content anyway let's limit size of the initial block to strict per-block maximum: seems pointless not to do that.

[cowtowncoder]

... even better. Just realized that due to buffer recycling, use of humongous block could even retain to problematic retention.
So this may block one possible real problem too, in addition to avoiding secondary issues.