Limit total size of one protobuf message

[matejcik]

Currently, core has a fixed 8kB buffer for receiving incoming messages, and tries to dynamically allocate more if the incoming message is larger.

For the vast majority of workflows, 8kB is wastefully large. The only exceptions that we know of are:

• uploading a custom homescreen -- the whole homescreen (16 kB max) must fit into a single message
• downloading list of resident credentials -- all credentials (at most 100) must fit into a single message

We should resolve these separately: introduce streaming for homescreen #1120 and probably for the credential list too.

It seems that 4096 bytes per message should be Enough For Everyone (tm). We need to test if that is really the case. If yes, we should:

• decrease the buffer size
• disable the dynamic allocation feature
• clearly document the fixed message size limit

[matejcik]

A MultisigRedeemScriptType can carry 3660 bytes of data, not counting the encoding overhead. This means that a standard TxInput message can end up very close to the 4kB limit.

6kB should be enough, but at that point maybe we can keep the 8kB buffer and just make it a hard limit.